SkPDF/docs: note that Sfntly subsetter is deprecated

Change-Id: I09ecee95653212268926d82739c0df3866d3a8d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253380
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>

include/docs/SkPDFDocument.h

@@ -164,6 +164,9 @@ struct Metadata {
     SkExecutor* fExecutor = nullptr;
 
     /** Preferred Subsetter. Only respected if both are compiled in.
+
+        The Sfntly subsetter is deprecated.
+
         Experimental.
     */
     enum Subsetter {