docs: Remove references to mmap in store-gateway (#8155)

* docs: Remove references to mmap in store-gateway

Per #4280, we removed use of mmap. This change removes references to it
in architecture documentation. It also removes references to features that
have been removed such as not using the bucket index.

Fixes #8145

Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>

* Code review

Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>

---------

Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>

docs/sources/mimir/references/architecture/components/store-gateway.md

@@ -15,12 +15,7 @@ On the read path, the [querier]({{< relref "./querier" >}}) and the [ruler]({{<
 ## Bucket index
 
 To find the right blocks to look up at query time, the store-gateway requires a view of the bucket in long-term storage.
-The store-gateway keeps the bucket view updated using one of the following options:
-
-- Periodically downloading the [bucket index]({{< relref "../bucket-index" >}}) (default)
-- Periodically scanning the bucket
-
-### Bucket index enabled (default)
+The store-gateway keeps the bucket view updated by periodically downloading the [bucket index]({{< relref "../bucket-index" >}}).
 
 To discover each tenant's blocks and block deletion marks, at startup, store-gateways fetch the [bucket index]({{< relref "../bucket-index" >}}) from long-term storage for each tenant that belongs to their [shard](#blocks-sharding-and-replication).
 
@@ -41,12 +36,6 @@ For example, if you're running the Grafana Mimir cluster in Kubernetes, you can
 
 For more information about the index-header, refer to [Binary index-header documentation]({{< relref "../binary-index-header" >}}).
 
-### Bucket index disabled
-
-When the bucket index is disabled, the overall workflow is still nearly the same.
-The difference occurs during the discovery phase of the blocks at startup, and during the periodic checks.
-Iterations over the entire long-term storage download `meta.json` metadata files while filtering out blocks that don't belong to tenants in their shard.
-
 ## Blocks sharding and replication
 
 The store-gateway uses blocks sharding to horizontally scale blocks in a large cluster.
@@ -125,11 +114,15 @@ Keeping the index-header on the local disk makes query execution faster.
 ### Index-header lazy loading
 
 By default, a store-gateway downloads the index-headers to disk and doesn't load them to memory until required.
-When required by a query, index-headers are memory-mapped and automatically released by the store-gateway after the amount of inactivity time you specify in `-blocks-storage.bucket-store.index-header.lazy-loading-idle-timeout` has passed.
+When required by a query, index-headers are loaded and automatically released by the store-gateway after the amount of inactivity time you specify in `-blocks-storage.bucket-store.index-header.lazy-loading-idle-timeout` has passed.
 
 Grafana Mimir provides a configuration flag `-blocks-storage.bucket-store.index-header.lazy-loading-enabled=false` to disable index-header lazy loading.
-When disabled, the store-gateway memory-maps all index-headers, which provides faster access to the data in the index-header.
-However, in a cluster with a large number of blocks, each store-gateway might have a large amount of memory-mapped index-headers, regardless of how frequently they are used at query time.
+When disabled, the store-gateway loads all index-headers at startup, which provides faster access to the data in the index-header when querying at the cost of longer startup times.
+However, in a cluster with a large number of blocks, each store-gateway might have a large amount of loaded index-headers, regardless of how frequently they are used at query time.
+
+When the index-header is loaded, only a portion of it is kept in memory to reduce memory usage.
+The rest of the index-header is read from disk as required.
+This requires that store-gateways have memory available to be used by the operating system for caching disk accesses.
 
 ## Caching