Clarify documentation of password quality check modules

Clarify documentation of password quality check modules

Be clearer in the info documentation that the part of the policy
name before the colon is the name of the module, not the static
string "module".  State explicitly that "builtin" can be used as the
module name to identify built-in policies.

Use the same terminology in kadm5_pwcheck(3) as the info documentation,
changing test-name to policy-name and vendor to module-name.  State
explicitly how the module name and policy name are used to select which
policies to run.

Rephrase a few sentences, add a paragraph break, and fix a few typos
for clarity.

Signed-off-by: Love Hornquist Astrand <lha@h5l.org>