[DOC] mail: clean some docstrings and helpers

As I was passing by I found some docstrings or helpers could be updated or
rephrased a bit more clearly. This is free as in free beers, without beers.

Some code about mail features (posting) may also be re-indented to ease
understanding of future modifications. Or just because we had to read
it and go through many files. Still free beers.

LINKS

Task ID-2477444
Prepares Task ID-2377974 (trace management cleaning task)
Prepares Task ID-2070632 (channel members main task)
Prepares Task ID-2419762 (channel members followup task)
COM PR odoo/odoo#67382
UPG PR odoo/upgrade#2245