DOC/MINOR: userlists: mention solutions to high cpu with hashes

DOC/MINOR: userlists: mention solutions to high cpu with hashes

This change adds a paragraph to the documentation regarding "userlists"
and the use of hashed password value.

It indicates what a user can do to address the high CPU cost of
having to calculate the hash at each request, such as reducing the
number of rounds or the cost complexity, if the algorithm allows for it.

I believe it is necessary to mention how the musl C library
impacts performance of hashing functions, as this has already led to a
few issues:

https://github.com/haproxy/haproxy/issues/1298
https://github.com/haproxy/haproxy/issues/2008
https://github.com/haproxy/haproxy/issues/2251

The performance impact is significant enough to mention it.

Acked-by: Lukas Tribus <lukas@ltri.eu>