Documenting the crypto modules

[Bruno Haible]

Hi Simon,

As mentioned in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-10/msg00221.html>,
the Gnulib documentation now has an appendix "Undocumented modules".
Currently, the crypto/* and crc* modules are listed there. Which means
that Gnulib users who don't know about the modules have little chance
of understanding whether the modules can be useful for their package.

How about documenting them? By that, I don't mean to copy the comments
from the .h file to a .texi file - that would be redundant. But rather,
to create a section, in chapter "Particular modules", that acts like
a road sign for these modules. It should answer the following questions
from the perspective of a Gnulib user:
  1) What kind of functionality does the group of modules provide?
  2) Why would I use the particular modules, as opposed to using
     libraries such as nettle, gnutls, or openssl?
  3) Once I have decided to use a module, is there something particular
     to watch out? (Build requirements, multithread-safety, etc.)

Bruno