docs: Document text_content field of our markdown test cases.

docs/markdown.md

@@ -60,6 +60,12 @@ this file:
   and `expected_marked_output` fields, and when `false`, should be
   replaced be one of those.  We plan to eliminate it once we're out of
   cases where it is `false`.
+* For mobile push notifications, we need a text version of the
+  rendered content, since the APNS and GCM push notification systems
+  don't support richer markup.  Mostly, this involves stripping HTML,
+  but there's some syntax we take special care with.  Tests for what
+  this plain-text version of content should be are stored in the
+  `text_content` field.
 
 If you're going to manually test some changes in the frontend Markdown
 implementation, the easiest way to do this is as follows: