Publish docs for rrect, picture, blendmode

Complete basic docs for SkRRect, SkPicture, SkBlendMode.
Add a new rule that checks the main description tense.
Check for spelling errors.

R=caryclark@google.com

Docs-Preview: https://skia.org/?cl=138542
Bug: skia:6898
Change-Id: Iba547873775a89f1d652be9b0219b84ffa8d0628
Reviewed-on: https://skia-review.googlesource.com/138542
Commit-Queue: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
diff --git a/docs/undocumented.bmh b/docs/undocumented.bmh
index 91e3a40..c4a65a1 100644
--- a/docs/undocumented.bmh
+++ b/docs/undocumented.bmh
@@ -137,14 +137,6 @@
 #Substitute HTML gray
 ##
 
-#Topic HTML_DarkGray 
-#Substitute HTML dark gray
-##
-
-#Topic HTML_LightGray 
-#Substitute HTML light gray
-##
-
 #Topic HTML_Silver 
 #Substitute HTML silver
 ##
@@ -614,6 +606,7 @@
 
 #Topic Premultiply
 #Alias Premultiplied ##
+#Alias Premultiplies ##
 ##
 
 #Topic Raster_Engine