asymptotic growth groups: fix docstrings related to categories

[dkrenn]

From cheuberg:

The docstring of GenericGrowthGroup (and the other growth groups) says "It has to be a subcategory of Join of Category of groups and Category of posets. This is also the default category if None is specified." which is a relict from the past.

Furthermore, we have

    # set everything up to determine category
    from sage.categories.sets_cat import Sets
    from sage.categories.posets import Posets
    from sage.categories.magmas import Magmas
    from sage.categories.additive_magmas import AdditiveMagmas

in the class GenericGrowthGroup which shows all these Categories in the documentation, similar as #20045.

Fix the docstrings!

CC: @behackl @cheuberg

Component: asymptotic expansions

Issue created by migration from https://trac.sagemath.org/ticket/20077