Decide how to document functions like is_trivial, is_irreducible with many different methods in a consistent way... docstrings vs. manual vs. REPL etc.

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[thofma]

Small remark from #4509

In case there is a common supertype (like RingElem, Ring, Ideal), we want to document the mathematical notion directly for this supertype. For example, that dim(R::Ring) should return the Krull dimension etc, without putting a dim docstring at every possible type, e.g., dim(::MPolyRing), dim(...). But as far as I understand, it is not clear if this makes sense for ==.

In the dim example, one has a "problem" with the online documentation, since one ideally wants to include the dim docstring in the documentation of polynomial rings, ideals, ..., etc, together with examples.