This commit adds a documentation note explaining the importance of
overriding equals() and hashCode() in custom UserDetails implementations.
The default SessionRegistryImpl in Spring Security uses an in-memory
ConcurrentMap<Object, Set<String>>, Map<String,SessionInformation> to
associate principals with sessions. If a custom UserDetails class does
not properly override equals() and hashCode(), user sessions may not
be tracked or matched correctly.
I believe this helps developers avoid subtle session management issues
when implementing custom authentication logic.
Signed-off-by: Gurunathan <129361658+Gurunathan16@users.noreply.github.com>