Modified comment on Asset meta data ignore to better explain usage. (#3249)

Unity.gitignore

@@ -10,7 +10,7 @@
 /[Ll]ogs/
 /[Mm]emoryCaptures/
 
-# Never ignore Asset meta data
+# Asset meta data should only be ignored when the corresponding asset is also ignored
 !/[Aa]ssets/**/*.meta
 
 # Uncomment this line if you wish to ignore the asset store tools plugin