drm/i915: Fix docbook descriptions for i915_gem_shrinker

Fixes the following htmldocs warning:
drivers/gpu/drm/i915/gem/i915_gem_shrinker.c:102: warning: Function parameter or member 'ww' not described in 'i915_gem_shrink'

Fixes: cf41a8f1 ("drm/i915: Finally remove obj->mm.lock.")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421120938.546076-1-maarten.lankhorst@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/gem/i915_gem_shrinker.c

@@ -70,6 +70,7 @@ static void try_to_writeback(struct drm_i915_gem_object *obj,
 
 /**
  * i915_gem_shrink - Shrink buffer object caches
+ * @ww: i915 gem ww acquire ctx, or NULL
  * @i915: i915 device
  * @target: amount of memory to make available, in pages
  * @nr_scanned: optional output for number of pages scanned (incremental)