drm/i915: kerneldoc for i915_gem_shrinker.c

And remove one bogus * from i915_gem_gtt.c since that's not a
kerneldoc there.

v2: Review from Chris:
- Clarify memory space to better distinguish from address space.
- Add note that shrink doesn't guarantee the freed memory and that
  users must fall back to shrink_all.
- Explain how pinning ties in with eviction/shrinker.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl
index 7a45775..f4976cd 100644
--- a/Documentation/DocBook/drm.tmpl
+++ b/Documentation/DocBook/drm.tmpl
@@ -4184,7 +4184,7 @@
       <sect2>
         <title>Buffer Object Eviction</title>
 	<para>
-	  This section documents the interface function for evicting buffer
+	  This section documents the interface functions for evicting buffer
 	  objects to make space available in the virtual gpu address spaces.
 	  Note that this is mostly orthogonal to shrinking buffer objects
 	  caches, which has the goal to make main memory (shared with the gpu
@@ -4192,6 +4192,17 @@
 	</para>
 !Idrivers/gpu/drm/i915/i915_gem_evict.c
       </sect2>
+      <sect2>
+        <title>Buffer Object Memory Shrinking</title>
+	<para>
+	  This section documents the interface function for shrinking memory
+	  usage of buffer object caches. Shrinking is used to make main memory
+	  available.  Note that this is mostly orthogonal to evicting buffer
+	  objects, which has the goal to make space in gpu virtual address
+	  spaces.
+	</para>
+!Idrivers/gpu/drm/i915/i915_gem_shrinker.c
+      </sect2>
     </sect1>
 
     <sect1>