Bug 1402026: Add caveat about suggested source_subtype. r=liuche

In the bug, we decided that it was okay to document this case because:
1) we didn't know the specific questions we were trying to answer
2) We were facing the 57 deadline

The alternative would be to change the behavior to perhaps the more intuitive
behavior where suggested sites will always be marked as "suggested" clicks but
note that there may be privacy concerns with that (in that there are a limited
number of suggested sites so we'd know the frequency that unique users might
visit the suggested sites).

MozReview-Commit-ID: GxQZzwoZ1nQ

--HG--
extra : rebase_source : 9c6697ef478a5ba08e1503d8360d1214419266fd

mobile/android/docs/activitystreamtelemetry.rst

@@ -65,6 +65,11 @@ Subtype indicates a reason an item which is being interacted with appeared in th
 - "suggested": a suggested top site, one of the default ones displayed when there's not enough browsing history available
 - "top": a frecency-based top site, based on browsing history. Neither "pinned" nor "suggested".
 
+**Important note on the suggested source_subtype:** "suggested" will only be returned the first time a user clicks on a
+particular suggested site: for all subsequent clicks on this same site, "top" will be returned (assuming the site is
+never pinned). The reason is that once a suggested site has been visited once, it becomes part of a user's history and
+is no longer suggested.
+
 Top Stories (Pocket) interactions
 ---------------------------------