Explain what to do with RestClient#request’s version param

There’s still a question of how this method is meant to behave if the user also
specifies an `X-Ably-Header` header in the `headers` parameter. But it’s a
question that equally applied before adding the `version` parameter, so I’ve
captured it in a separate issue #144.

Same comment as 69e1973 re needing to update "replaced in" spec version before
merging integration branch into `main` applies here too.

textile/features.textile

@@ -93,7 +93,8 @@ h3(#restclient). RestClient
 * @(RSC21)@ @RestClient#push@ attribute provides access to the @Push@ object that was instantiated with the @ClientOptions@ provided in the @RestClient@ constructor
 * @(RSC22)@ @RestClient#batch@ attribute provides access to the @BatchOperations@ object that was instantiated with the @ClientOptions@ provided in the @RestClient@ constructor
 * @(RSC7)@ Sends REST requests over HTTP and HTTPS to the REST endpoint @rest.ably.io@
-** @(RSC7a)@ The @X-Ably-Version@ HTTP header must be included in all REST requests to Ably endpoints. The value to be sent is defined by "@CSV2@":#CSV2.
+** @(RSC7a)@ This clause has been replaced by "@RSC7e@":#RSC7e. It was valid up to and including specification version @2.0@.
+** @(RSC7e)@ The @X-Ably-Version@ HTTP header must be included in all REST requests to Ably endpoints. The value to be sent is defined by "@CSV2@":#CSV2, except in the case where the user directly calls @RestClient#request@, in which case the value to be sent is defined by "@RSC19f1@":#RSC19f1.
 ** @(RSC7b)@ (Please note this clause and the associated header have now been superseded by "RCS7d":#RSC7d) The header @X-Ably-Lib: [lib][.optional variant]?-[version]@ should be included in all REST requests to the Ably endpoint where @[lib]@ is the name of the library such as @js@ for @ably-js@, @[.optional variant]@ is an optional library variant, such as @laravel@ for the @php@ library, which is always delimited with a period such as @php.laravel@, and where @[version]@ is the full client library version using "Semver":http://semver.org/ such as @1.0.2@. For example, the 1.0.0 version of the JavaScript library would use the header @X-Ably-Lib: js-1.0.0@.
 *** @(RSC7b1)@ When it is not possible to send the @X-Ably-Lib@ header, such as for @JSONP@ requests, the library version should be sent as a query param such as @lib=js-1.0.0@
 ** @(RSC7c)@ If the @addRequestIds@ client option is enabled, every REST request to Ably should include in a @request_id@ query string parameter a random string obtained by url-safe base64-encoding a sequence of at least 9 bytes obtained from a source of randomness. This request ID must remain the same if a request is retried to a fallback host per @RSC15@. Any log messages associated with the request should include the request ID. If the request fails, the request ID must be included in the @ErrorInfo@ returned to the user.
@@ -149,6 +150,7 @@ h3(#restclient). RestClient
 * @(RSC19)@ @RestClient#request@ function is provided as a convenience for customers who wish to use REST API functionality that is either not documented or is not included in the API for our client libraries. The REST client library provides a function to issue HTTP requests to the Ably endpoints with all the built in functionality of the library such as authentication, paging, fallback hosts, MsgPack and JSON support etc. The function:
 ** @(RSC19a)@ This clause has been replaced by "@RSC19f@":#RSC19f. It was valid up to and including specification version @2.0@.
 ** @(RSC19f)@ Method signature is @request(String method, String path, Int version, Dict<String, String> params?, JsonObject | JsonArray body?, Dict<String, String> headers?) -> HttpPaginatedResponse@ with arguments: @method@ is a valid "HTTP verb":https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html (must support @"GET"@, @"POST"@, and @"PUT"@, should support @"PATCH"@ and @"DELETE", may support others); @path@ is the path component of the URL such as @"/channels"@; @params@ and @headers@ are optional arguments containing key-value pairs of strings (multi-valued headers are not supported) that will respectively result in querystring params and HTTP headers being added to the request (the argument types can be idiomatic for the language such as @Object@ in the case of JavaScript); @body@ is an optional @JsonObject@ or @JsonArray@ like object argument that can be easily serialized to MsgPack or JSON
+*** @(RSC19f1)@ The @version@ parameter specifies the protocol version that the Ably service should use when interpreting the request. The library must include an @X-Ably-Version@ header in the request that it sends, with value given by this argument.
 ** @(RSC19b)@ All requests will unconditionally use the default authentication mechanism configured for the REST client i.e. basic or token authentication (see "Auth":#rest-auth)
 ** @(RSC19c)@ The library will configure the @Accept@ and @Content-Type@ type headers to reflect whether the client is configured to use a binary or JSON based protocol (see "RSC8":#RSC8). All requests are encoded and decoded into Json or MsgPack as appropriate automatically by the library. Binary @body@ payloads are not supported at this time
 ** @(RSC19d)@ @request@ method returns an @HttpPaginatedResponse@ object that inherits from the @PaginatedResult@ object to provide details on the response plus paging support where applicable. See "HP1":#HP1 for more details