Fix ast_str_truncate signedness warning and documentation.

Review: https://reviewboard.asterisk.org/r/1594
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Merged revisions 346144 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 346145 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@346146 65c4cc65-6c06-0410-ace0-fbb531ad65f3