docs: ABI: ABI documentation for procfs attribute files used by multiple LSMs

Provide basic ABI descriptions for the process attribute entries
that are shared between multiple Linux security modules.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Link: https://lore.kernel.org/r/30c36660-3694-0c0d-d472-8f3b3ca4098e@schaufler-ca.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/procfs-attr-current

+What:		/proc/*/attr/current
+Contact:	linux-security-module@vger.kernel.org,
+		selinux@vger.kernel.org,
+		apparmor@lists.ubuntu.com
+Description:	The current security information used by a Linux
+		security module (LSM) that is active on the system.
+		The details of permissions required to read from
+		this interface and hence obtain the security state
+		of the task identified is LSM dependent.
+		A process cannot write to this interface unless it
+		refers to itself.
+		The other details of permissions required to write to
+		this interface and hence change the security state of
+		the task identified are LSM dependent.
+		The format of the data used by this interface is LSM
+		dependent.
+		SELinux, Smack and AppArmor provide this interface.
+Users:		SELinux user-space
+		Smack user-space
+		AppArmor user-space

Documentation/ABI/testing/procfs-attr-exec

+What:		/proc/*/attr/exec
+Contact:	linux-security-module@vger.kernel.org,
+		selinux@vger.kernel.org,
+		apparmor@lists.ubuntu.com
+Description:	The security information to be used on the process
+		by a Linux security module (LSM) active on the system
+		after a subsequent exec() call.
+		The details of permissions required to read from
+		this interface and hence obtain the security state
+		of the task identified is LSM dependent.
+		A process cannot write to this interface unless it
+		refers to itself.
+		The other details of permissions required to write to
+		this interface and hence change the security state of
+		the task identified are LSM dependent.
+		The format of the data used by this interface is LSM
+		dependent.
+		SELinux and AppArmor provide this interface.
+Users:		SELinux user-space
+		AppArmor user-space

Documentation/ABI/testing/procfs-attr-prev

+What:		/proc/*/attr/prev
+Contact:	linux-security-module@vger.kernel.org,
+		selinux@vger.kernel.org,
+		apparmor@lists.ubuntu.com
+Description:	The security information used on the process by
+		a Linux security module (LSM) active on the system
+		prior to the most recent exec() call.
+		The details of permissions required to read from
+		this interface is LSM dependent.
+		A process cannot write to this interface unless it
+		refers to itself.
+		The other details of permissions required to write to
+		this interface are LSM dependent.
+		The format of the data used by this interface is LSM
+		dependent.
+		SELinux and AppArmor provide this interface.
+Users:		SELinux user-space
+		AppArmor user-space
+