followup to Issue 9180
Document 64bit registers and SIMD instructions
Add note about symbol names which match keywords.

If symbol's name would be a keyword, and it is undesirable to pick
another name, then the way to make it legal is to append an underscore
to it, which we've been doing. But apparently it didn't make it into the
style guide. So, this adds it.
Merge pull request #227 from jmdavis/dstyle

Add note about enum member names which match keywords.
Merge pull request #228 from Govelius/master

Update iasm docs
add in 2.061 changelog
Make remote explicit in git fetch