docs: add missing subscripts in `sign` definition

[mdhaber]

Looked like these subscripts were omitted.

LMK if this should be adjusted in 2012/2013 standards, but I think the intent is obvious enough that it doesn't need to be fixed in old versions.

[kgryte]

Thanks for flagging this. Yes, if you wouldn't mind backporting to previous spec versions, that would be helpful.

[mdhaber]

Thanks @kgryte. I've added the subscripts to past versions, too.