[container.insert.return] Fix description of insert-return-type (#8030)

Use the usual phrasing "X is for exposition only" when X is an exposition-only construct.

Author: adr26

source/containers.tex

@@ -2622,7 +2622,7 @@
 \end{codeblock}
 
 \pnum
-The name \exposid{insert-return-type} is exposition only.
+The name \exposid{insert-return-type} is for exposition only.
 \exposid{insert-return-type} has the template parameters,
 data members, and special members specified above.
 It has no base classes or members other than those specified.