Annotate the unusual use of "$callback()" in "xsprintf()"

Summary: Ref T13588. See D21500. This syntax is unusual and there are some hidden complexities involved; annotate them. See D21500 for more discussion.

Test Plan: Read text, reviewed D21500.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21541