Clarify use of validation tools to warn about overly constrained uses of content and processor profile (#506)

Author: palemieux

imsc1/spec/ttml-ww-profiles.html

@@ -452,6 +452,13 @@ <h3>General</h3>
       it is not possible to construct a <a>Document Instance</a> that results in the presentation of both text data and image
       data.</p>
 
+      <p class='note'>Authors can use TTML content verification tools to detect
+      and warn about overly constrained uses of content and processor profile
+      attribute vocabulary, e.g. warn if (a) a document simultaneously (i)
+      specifies conformance to content profile X and (ii) requires use of
+      processor profile Y, and (b) profiles X and Y are semantically
+      incompatible.</p>
+
       <p>In applications that require subtitle/caption content in image form to be simultaneously available in text form, two
       distinct <a data-lt="Document Instance">Document Instances</a>, one conforming to the <a>Text Profile</a> and the other
       conforming to the <a>Image Profile</a>, SHOULD be offered. In addition, the <a>Text Profile</a> <a>Document Instance</a>