[backport#16887 2/3] Add documenting comments to spanparsing.h

Summary:
https://github.com/bitcoin/bitcoin/pull/16887/commits/5e69aeec3f2a0fafd5e591b7222716f00145761d

Depends on D6877

Partial backport of Core PR16887

Test Plan:

read the comments

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6878