man: say runtime dir has standard not best features (#43915)

The XDG spec only requires "The directory MUST be fully-featured by the
standards of the operating system". Our previous wording promised the
"greatest" features across the entire OS. Trying to rely on that
resulted in an error while trying to enable `chattr +F` on tmpfs
/run/user/1000, whereas this feature is supported for my /home. This PR
narrows the wording to not exceed the XDG spec.

man/pam_systemd.xml

@@ -335,8 +335,8 @@
         directory shall be used for runtime file system objects such
         as <constant>AF_UNIX</constant> sockets, FIFOs, PID files and
         similar. It is guaranteed that this directory is local and
-        offers the greatest possible file system feature set the
-        operating system provides. For further details, see the <ulink
+        fully-featured by the standards of the operating system. For
+        further details, see the <ulink
         url="https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
         Base Directory Specification</ulink>. <varname>$XDG_RUNTIME_DIR</varname>
         is not set if the current user is not the original user of the session.</para></listitem>