Weaken statement about strong consistency guarantees

Strong consistency for Praefect/Gitaly isn't yet available for all
mutating RPCs, but the documentation makes it sound like that. Let's
mention that this currently is implemented for a subset of RPCs, only,
to avoid any confusion or wrong expectations.

doc/administration/gitaly/praefect.md

@@ -35,7 +35,8 @@ The availability objectives for Gitaly clusters are:
   Writes are replicated asynchronously. Any writes that have not been replicated
   to the newly promoted primary are lost.
 
-  [Strong consistency](#strong-consistency) can be used to improve this to "no loss".
+  [Strong consistency](#strong-consistency) can be used to avoid loss in some
+  circumstances.
 
 - **Recovery Time Objective (RTO):** Less than 10 seconds.
 
@@ -886,8 +887,8 @@ after the write to the primary Gitaly node has happened.
 Praefect can instead provide strong consistency by creating a transaction and writing
 changes to all Gitaly nodes at once. Strong consistency is currently in
 [alpha](https://about.gitlab.com/handbook/product/#alpha-beta-ga) and not enabled by
-default. For more information, see the
-[strong consistency epic](https://gitlab.com/groups/gitlab-org/-/epics/1189).
+default. If enabled, transactions are only available for a subset of RPCs. For more
+information, see the [strong consistency epic](https://gitlab.com/groups/gitlab-org/-/epics/1189).
 
 To enable strong consistency: