man/login.defs.5.xml: clarify documentation for multi-component usage
The login.defs configuration file is used by multiple system components
(shadow-utils, PAM, util-linux), but the documentation was unclear about
this reality. This led to confusion about which parameters are relevant
on different system configurations.
Add explanatory paragraph clarifying that login.defs parameters may be
used by shadow-utils, PAM, and other system components, with behaviour
depending on system configuration and enabled authentication mechanisms.