Fix the return description for `switch_to_blog()` to reflect that it always returns true.

Props jeremyfelt.
Fixes #25328.

Built from https://develop.svn.wordpress.org/trunk@27347


git-svn-id: http://core.svn.wordpress.org/trunk@27199 1a063a9b-81f0-0310-95a4-ce76da25c4cd