doc: cgroup: improve formatting of references

Annotate references to other documents to make them clickable.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20200228000653.1572553-6-kuba@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/accounting/psi.rst

+.. _psi:
+
 ================================
 PSI - Pressure Stall Information
 ================================

Documentation/admin-guide/cgroup-v1/index.rst

+.. _cgroup-v1:
+
 ========================
 Control Groups version 1
 ========================

Documentation/admin-guide/cgroup-v2.rst

@@ -9,7 +9,7 @@ This is the authoritative documentation on the design, interface and
 conventions of cgroup v2.  It describes all userland-visible aspects
 of cgroup including core and specific controller behaviors.  All
 future changes must be reflected in this document.  Documentation for
-v1 is available under Documentation/admin-guide/cgroup-v1/.
+v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgroup-v1>`.
 
 .. CONTENTS
 
@@ -1023,7 +1023,7 @@ All time durations are in microseconds.
 	A read-only nested-key file which exists on non-root cgroups.
 
 	Shows pressure stall information for CPU. See
-	Documentation/accounting/psi.rst for details.
+	:ref:`Documentation/accounting/psi.rst <psi>` for details.
 
   cpu.uclamp.min
         A read-write single value file which exists on non-root cgroups.
@@ -1391,7 +1391,7 @@ PAGE_SIZE multiple when read back.
 	A read-only nested-key file which exists on non-root cgroups.
 
 	Shows pressure stall information for memory. See
-	Documentation/accounting/psi.rst for details.
+	:ref:`Documentation/accounting/psi.rst <psi>` for details.
 
 
 Usage Guidelines
@@ -1631,7 +1631,7 @@ IO Interface Files
 	A read-only nested-key file which exists on non-root cgroups.
 
 	Shows pressure stall information for IO. See
-	Documentation/accounting/psi.rst for details.
+	:ref:`Documentation/accounting/psi.rst <psi>` for details.
 
 
 Writeback