[docs] Expand the pre-merge testing description a bit

Core changes are:

    Be explicit about desired balance between missing true positives and reporting false positives.
    Mention the opt-out mechanism.
    Provide links to background, and give description of who to contact if needed.

Differential Revision: https://reviews.llvm.org/D110873

Author: preames

llvm/docs/Phabricator.rst

@@ -181,7 +181,12 @@ branch - thus the term *pre-merge testing*.
 
 When a user uploads a patch to Phabricator, Phabricator triggers the checks and
 then displays the results. This way bugs in a patch are contained during the 
-code review stage and do not pollute the main branch. 
+code review stage and do not pollute the main branch.
+
+Our goal with pre-merge testing is to report most true problems while strongly
+minimizing the number of false positive reports.  Our goal is that problems
+reported are always actionable.  If you notice a false positive, please report
+it so that we can identify the cause.
 
 If you notice issues or have an idea on how to improve pre-merge checks, please 
 `create a new issue <https://github.com/google/llvm-premerge-checks/issues/new>`_ 
@@ -217,6 +222,25 @@ the unit test to see the details:
 
   .. image:: Phabricator_premerge_unit_tests.png
 
+Opting Out
+^^^^^^^^^^
+
+In case you want to opt-out entirely of pre-merge testing, add yourself to the
+`OPT OUT project <https://reviews.llvm.org/project/view/83/>`_.  If you decide
+to opt-out, please let us know why, so we might be able to improve in the future.
+
+Operational Details
+^^^^^^^^^^^^^^^^^^^
+
+The code responsible for running the pre-merge flow can be found in the external
+repository  `https://github.com/google/llvm-premerge-checks`_.  For enhancement
+ideas and most bugs, please file an issue on said repository.  For immediate
+operational problems, the point of contact is
+`Mikhail Goncharov <mailto:[email protected]>`_.
+
+Background on the pre-merge infrastructure can be found in `this 2020 DevMeeting
+talk <https://llvm.org/devmtg/2020-09/slides/Goncharov-Pre-merge_checks.pdf>`_
+
 Committing a change
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