Clarify function descriptions for get_gmt_from_date() and get_date_from_gmt()

[johnbillion]

The descriptions for these functions don't make sense, which doesn't help developers who are trying to make use of them.

[johnbillion]

48650.diff​ is my proposed patch.

cc @rarst can you confirm that these descriptions are accurate please?

[Rarst]

Looks good, one small thing is that I've been trying to move towards saying UTC, rather than GMT time zone in documentation.

While colloquially these are synonyms, speaking technically GMT is a different historical thing and what we actually use and operate with on PHP level is UTC.

I leave existing gmt in code and we have it hardwired into API and database schema, but on documentation level I feel UTC is more appropriate to describe what is going on.

[Rarst]

Also returns could probably use a comma: Formatted version of the date provided, in UTC timezone., otherwise it can be read different way.

[SergeyBiryukov]

In 46958:

Docs: Clarify descriptions for get_gmt_from_date() and get_date_from_gmt().

Props johnbillion, Rarst.
Fixes #48650.