Update stringfield documentation for removed second va_list in favor of va_copy.

In r320946, the second va_list that was passed to ast_string_field_build_va
and friends, was removed. This patch updates the documentation to reflect that.
........

Merged revisions 357620 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@357621 65c4cc65-6c06-0410-ace0-fbb531ad65f3