Document more X509_STORE functions

X509_STORE_set_verify_cb_func.pod has documentation for various callbacks
and function pointers that can be set and retrieved, but neither it nor
X509_STORE_new has much documentation for the actual purpose and usage
of X509_STORE objects.  Remedy this disparity with new documentation
for adding certificates and CRLs, expected usage, and for modifying
the default verifification behavior.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3958)