Need spec changes for "M" prefix on monthCode

[justingrant]

#1341 (adding "M" prefix to monthCode) didn't contain any spec changes because I didn't know how to document string concatenation or splitting in spec language. Could someone who is better at spec-ese than I am take a crack at it?

[cjtenny]

After the meeting tomorrow I'll knock it out for temporal, since temporal AFAIK should only contain the ISO month code specs - quite quick!

[justingrant]

Following up from #1245 (comment), we may want to include the one-line spec change to ToTemporalMonthDay (3.j.i) in the fix to this issue.

[cjtenny]

I have commits for both of those and other issues that I've been scared to push lest they be perceived as API changes. I asked a few people for feedback and the suggestion was to push them separately and highlight that they're small & not {bugfixes, to-be-standardized but not-262, etc}.