Document RSA operations (hashicorp#19377)

Also clarify hash function choices.

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

website/content/docs/secrets/transit/index.mdx

@@ -99,6 +99,12 @@ supports HMAC, and behaves identically to other algorithms with
 respect to the HMAC operations but supports key import. By default,
 the HMAC key type uses a 256-bit key.
 
+RSA operations use one of the following methods:
+
+ - OAEP (encrypt, decrypt), with SHA-256 hash function and MGF,
+ - PSS (sign, verify), with configurable hash function also used for MGF, and
+ - PKCS#1v1.5: (sign, verify), with configurable hash function.
+
 ## Convergent Encryption
 
 Convergent encryption is a mode where the same set of plaintext+context always