Add default values for rc_invites per_issuer to docs (#17347)

A simple change to update the docs where default values were missing.

### Pull Request Checklist

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* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
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The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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---------

Co-authored-by: Kim Brose <2803622+HarHarLinks@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/17347.doc

@@ -0,0 +1 @@
+Add default values for `rc_invites.per_issuer` to docs.

docs/usage/configuration/config_documentation.md

@@ -1759,8 +1759,9 @@ rc_3pid_validation:
 ### `rc_invites`
 
 This option sets ratelimiting how often invites can be sent in a room or to a
-specific user. `per_room` defaults to `per_second: 0.3`, `burst_count: 10` and
-`per_user` defaults to `per_second: 0.003`, `burst_count: 5`.
+specific user. `per_room` defaults to `per_second: 0.3`, `burst_count: 10`,
+`per_user` defaults to `per_second: 0.003`, `burst_count: 5`, and `per_issuer` 
+defaults to `per_second: 0.3`, `burst_count: 10`.
 
 Client requests that invite user(s) when [creating a
 room](https://spec.matrix.org/v1.2/client-server-api/#post_matrixclientv3createroom)