[std] Punctuation after "OK" example comments

[jensmaurer]

I thought the policy for example comments "// OK, explanation" was to use a comma. However, the statistics are not in my favor:

$ grep "// OK," *.tex | wc -l
138
$ grep "// OK:" *.tex | wc -l
251

Asking for confirmation of the policy before providing a large-scale fix.

Incidentally related to #3126.

[jensmaurer]

Editorial meeting: Confirmed. Some of the colons should become semicolons instead of commas.