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The web page about changes in 4.7 (http://gcc.gnu.org/gcc-4.7/changes.html) mentions the option -mms-bitfields, but manual for 4.7 does not document it
(at least the chapter "Option summary" misses it).
There is actually a fairly detailed description of what this option does in conjunction with the corresponding variable attributes in extend.texi. I'll hack up a patch to add some cross-references.
Fixed in commit 230467 (sorry, I screwed up the PR # in the commit message so it didn't get recorded here automatically).