Remove mention of multisets from EnumeratedSets

[mkoeppe]

... as it does not make sense as a subcategory of Sets

Instead if multisets are needed anywhere, they should go through the values() or items() methods of EnumeratedFamilies (#34340).

For enumerated sets of non-hashable elements, we need to provide (slow) methods for cardinality, containment, etc. that use comparisons only (see #23324).

Depends on #34340

CC: @tscrim

Component: categories

Issue created by migration from https://trac.sagemath.org/ticket/34389

[mkoeppe]

Dependencies: #34340