Documentation improvements for some ulong_extras functions

[vneiger]

Some functions in ulong_extras have outdated or inaccurate descriptions, see
#2051 (comment)

Also, it is slightly misleading that some _preinv-suffixed functions require a normalized input (e.g. n_mulmod_preinv, udiv_qrnnd_preinv) while most others don't. A solution may be to add _prenorm or something like this to functions that require normalization, and keep the current names for ones that work in general. E.g. we would have both udiv_qrnnd_preinv (general) and udiv_qrnnd_preinv_prenorm; both returning the same thing in the normalized case, so this should not break existing code. Comments/ideas about this are welcome. If no such renaming is done, we should at least add some warning about the possible normalization requirements in the introductory paragraphs of the ulong_extras documentation.