Add commentary on the compatibility decomposition in the normalizer

src/hb-ot-shape-normalize.cc

@@ -63,10 +63,22 @@
  *
  *   - When a font does not support a character but supports its decomposition,
  *     well, use the decomposition (preferring the canonical decomposition, but
- *     falling back to the compatibility decomposition if necessary).
+ *     falling back to the compatibility decomposition if necessary).  The
+ *     compatibility decomposition is really nice to have, for characters like
+ *     ellipsis, or various-sized space characters.
  *
- *   - The Indic shaper requests decomposed output.  This will handle splitting
- *     matra for the Indic shaper.
+ *   - The complex shapers can customize the compose and decompose functions to
+ *     offload some of their requirements to the normalizer.  For example, the
+ *     Indic shaper may want to disallow recomposing of two matras.
+ *
+ *   - We try compatibility decomposition if decomposing through canonical
+ *     decomposition alone failed to find a sequence that the font supports.
+ *     We don't try compatibility decomposition recursively during the canonical
+ *     decomposition phase.  This has minimal impact.  There are only a handful
+ *     of Greek letter that have canonical decompositions that include characters
+ *     with compatibility decomposition.  Those can be found using this command:
+ *
+ *     egrep  "`echo -n ';('; grep ';<' UnicodeData.txt | cut -d';' -f1 | tr '\n' '|'; echo ') '`" UnicodeData.txt
  */
 
 static void