Document generic polynomial context macros (and make them consistent?)

[rburing]

We currently have:

https://github.com/flintlib/flint/blob/3473f0cac16a64fea0f0b8fc66bb79ab71989999/src/gr.h#L1418-L1419

https://github.com/flintlib/flint/blob/3473f0cac16a64fea0f0b8fc66bb79ab71989999/src/gr.h#L1468-L1470

https://github.com/flintlib/flint/blob/3473f0cac16a64fea0f0b8fc66bb79ab71989999/src/gr.h#L1472-L1474

https://github.com/flintlib/flint/blob/3473f0cac16a64fea0f0b8fc66bb79ab71989999/src/gr_mpoly.h#L57-L60

(And later we will have macros for Ore polynomials.)

These are all useful, but only the gr_mpoly ones are documented.

Also the naming scheme of the gr_mpoly macros is inconsistent with the others.