Bug 1475256 - Add documentation for robocop code coverage. r=nalexander,gbrown

Depends on D4145

Differential Revision: https://phabricator.services.mozilla.com/D4146

--HG--
extra : moz-landing-system : lando

mobile/android/docs/testcoverage.rst

@@ -115,3 +115,19 @@ true`` to ``am instrument``.
 
 .. _GeckoView: https://wiki.mozilla.org/Mobile/GeckoView
 .. _geckoview-junit: https://developer.mozilla.org/en-US/docs/Mozilla/Geckoview-Junit_Tests
+
+
+Code coverage for Robocop UI tests
+==================================
+
+Robocop_ provides UI-level testing for Fennec. The tests use the same
+automation scripts as geckoview-junit, only differing by the fact that they are
+run by ``runrobocop.py`` instead of ``runjunit.py``.
+
+The only notable difference from the geckoview-junit tests is that robocop
+tests are run with one ``am instrument`` call per test class. This means that
+one ``robocop-coverage-<test name>.ec`` file will be generated for each test
+class. After the tests finish, ``jacoco-cli`` will be called on all the
+resulting coverage data files.
+
+.. _Robocop: https://wiki.mozilla.org/Auto-tools/Projects/Robocop