Add two cache warning

Change-Id: Ib1b3e8598b5e61a866446ec7c007672d089f634b
Reviewed-on: https://skia-review.googlesource.com/157525
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>

src/core/SkGlyphRunPainter.h

@@ -161,6 +161,8 @@ void SkGlyphRunListPainter::forEachMappedDrawableGlyph(
     }
 }
 
+// Beware! The following code will end up holding two glyph caches at the same time, but they
+// will not be the same cache (which would cause two separate caches to be created).
 template <typename PerPathT>
 void SkGlyphRunListPainter::drawGlyphRunAsPathWithARGBFallback(
         SkGlyphCacheInterface* pathCache, const SkGlyphRun& glyphRun,