Correct documentation about registration_shared_secret_path (#15168)

* Correct documentation about registration_shared_secret_path

* Create 15168.doc

* Update changelog.d/15168.doc

---------

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

changelog.d/15168.doc

@@ -0,0 +1 @@
+Correct the description of the behavior of `registration_shared_secret_path` on startup.

docs/usage/configuration/config_documentation.md

@@ -2227,8 +2227,8 @@ allows the shared secret to be specified in an external file.
 
 The file should be a plain text file, containing only the shared secret.
 
-If this file does not exist, Synapse will create a new signing
-key on startup and store it in this file.
+If this file does not exist, Synapse will create a new shared
+secret on startup and store it in this file.
 
 Example configuration:
 ```yaml