Comments: More comments on functions/globals in standard.h.

Summary: Backport of Core PR11058

Test Plan:

make check

Reviewers: #bitcoin_abc, Mengerian, markblundeberg

Reviewed By: #bitcoin_abc, Mengerian, markblundeberg

Subscribers: Fabien, markblundeberg, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D3113