I have exactly the same question and I haven't been able to find more detailed information on the XT:A:M tag from BWA.
I created a simple example (on my blog at
http://davetang.org/muse/2014/07/24/...red-end-reads/; specifically the "RESCUING AMBIGUOUSLY MAPPED READS" section) trying to obtain the XT:A:M tag by aligning a uniquely- and multi-mapping pair but I got the XT:A:U and XT:A:R tags instead.