cmd/go/doc.go: update translation

go/build: mention 'tag' as an alias for 'build constraint'
cmd/go, go/build: support .m files
cmd/go: document new -exec flag on run/test
cmd/go: make the default coverage mode -atomic if -race is set
cmd/go: fix typo in help doc
cmd/go: allow use of Context in 'go list -f'
cmd/go: reapply doc change from CL 60590044.
cmd/go: accept build flags in clean and list
cmd/go: link SWIG objects directly rather than using a shared library
cmd/go: document file types

https://code.google.com/p/go/source/browse/src/cmd/go/doc.go?r=7b833ef60b534ade4a58c40066e5874ca665c7d5