layers: Reword from cryptic abbreviations to readable words

Expand abbreviations to improve readability:
DS=depth/stencil
DS=descriptor set
CB=command buffer
VS=vertex shader
FS=fragment shader
VTX=vertex
PSO=pipeline state object

Change-Id: I48fe917aac50d1911fe2a3aa1ed2c739637c7645
diff --git a/layers/descriptor_sets.cpp b/layers/descriptor_sets.cpp
index 1e22f82..1148995 100644
--- a/layers/descriptor_sets.cpp
+++ b/layers/descriptor_sets.cpp
@@ -780,7 +780,7 @@
         }
         break;
     default:
-        // For other layouts if the source is ds image, both aspect bits must not be set
+        // For other layouts if the source is depth/stencil image, both aspect bits must not be set
         if (ds) {
             if (aspect_mask & VK_IMAGE_ASPECT_DEPTH_BIT) {
                 if (aspect_mask & VK_IMAGE_ASPECT_STENCIL_BIT) {