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

[rburing]

We currently have:

flint/src/gr.h

Lines 1418 to 1419 in 3473f0c

	#define POLYNOMIAL_CTX(ring_ctx) ((polynomial_ctx_t *)((ring_ctx)))
	#define POLYNOMIAL_ELEM_CTX(ring_ctx) (POLYNOMIAL_CTX(ring_ctx)->base_ring)

flint/src/gr.h

Lines 1468 to 1470 in 3473f0c

	#define SERIES_CTX(ring_ctx) ((series_ctx_t *)((ring_ctx)))
	#define SERIES_ELEM_CTX(ring_ctx) (SERIES_CTX(ring_ctx)->base_ring)
	#define SERIES_SCTX(ring_ctx) (&(((series_ctx_t *)((ring_ctx)))->sctx))

flint/src/gr.h

Lines 1472 to 1474 in 3473f0c

	#define SERIES_MOD_CTX(ring_ctx) ((series_mod_ctx_t *)((ring_ctx)))
	#define SERIES_MOD_ELEM_CTX(ring_ctx) (SERIES_MOD_CTX(ring_ctx)->base_ring)
	#define SERIES_MOD_N(ring_ctx) (SERIES_MOD_CTX(ring_ctx)->n)

flint/src/gr_mpoly.h

Lines 57 to 60 in 3473f0c

	#define GR_MPOLY_MCTX(ctx) (((_gr_mpoly_ctx_struct *) (ctx->data))->mctx)
	#define GR_MPOLY_CCTX(ctx) (((_gr_mpoly_ctx_struct *) (ctx->data))->cctx)
	#define GR_MPOLY_VARS(ctx) (((_gr_mpoly_ctx_struct *) (ctx->data))->vars)
	#define GR_MPOLY_NVARS(ctx) (GR_MPOLY_MCTX(ctx)->nvars)

(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.