Manpages improvement for subordinate IDs.

Manpages improvement for subordinate IDs.

	* man/newusers.8.xml: Include documentation of SUB_GID_MIN,
	SUB_GID_MAX, SUB_GID_COUNT, SUB_UID_MIN, SUB_UID_MAX,
	SUB_UID_COUNT.
	* man/useradd.8.xml: Likewise.
	* man/usermod.8.xml: Likewise.
	* man/newusers.8.xml: Document usage of /etc/subgid /etc/subuid.
	* man/useradd.8.xml: Likewise.
	* man/userdel.8.xml: Likewise.
	* man/usermod.8.xml: Likewise.
	* man/newusers.8.xml: Add references to subgid(5) and subuid(5).
	* man/useradd.8.xml: Likewise.
	* man/userdel.8.xml: Likewise.
	* man/subgid.5.xml: Sort references alphabetically.
	* man/subuid.5.xml: Likewise.
	* man/subgid.5.xml: Add references to newusers(8), useradd(8),
	userdel(8), usermod(8), user_namespaces(7).
	* man/subuid.5.xml: Likewise.