Doc: Clean up musttail attribute docs some more

Doc: Clean up musttail attribute docs some more

Looking over the recently-committed change to the musttail attribute
documentation, it appears the comment in the last example was a paste-o,
as it does not agree with either what the similar example in the
-Wmaybe-musttail-local-addr documentation says, or the actual behavior
observed when running the code.

In addition, the entire section on musttail was in need of copy-editing
to put it in the present tense, avoid reference to "the user", etc.  I've
attempted to clean it up here.

gcc/ChangeLog
	* doc/extend.texi (Statement Attributes): Copy-edit the musttail
	attribute documentation and correct the comment in the last
	example.