Add documentation for Status bot filters

BUG=skia:4602
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1489583002

Review URL: https://codereview.chromium.org/1489583002

site/dev/testing/buildbot.md

@@ -85,6 +85,18 @@ For more detail, you can click on an individual cell to get a summary of the
 steps which ran for that build.  You can also click one of the white bars at
 the top of each column to see a summary of recent builds for a given builder.
 
-
+The status page has several filters which can be used to show only a subset of
+bots:
+
+* Interesting: Bots which have both successes and failures within the visible
+  commit window.
+* Failures: Bots which have failures within the visible commit window.
+* Comments: Bots which have comments.
+* Failing w/o comment: Bots which have failures within the visible commit window
+  but have no comments.
+* All: Display all bots.
+* Search: Enter a search string. Substrings and regular expressions may be
+  used, per the Javascript String Match() rules:
+  http://www.w3schools.com/jsref/jsref_match.asp