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

[reames]

Summary

Core changes are:

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

[goncharov]

thanks

llvm/docs/Phabricator.rst

188

s/let//

llvm/docs/Phabricator.rst

 -----------------
 
 The pre-merge tests are a continuous integration (CI) workflow. The workflow 
 checks the patches uploaded to Phabricator before a user merges them to the main 
 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 let
+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>`_ 
 or give a ❤️ to an existing one.
 
 Requirements
 ^^^^^^^^^^^^
 
 To get a patch on Phabricator tested, the build server must be able to apply the

 The CI will compile and run tests, run clang-format and clang-tidy on lines
 changed.
 
 If a unit test failed, this is shown below the build status. You can also expand
 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:[e-mail]>`_.
+
+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
 -------------------
 
 Once a patch has been reviewed and approved on Phabricator it can then be
 committed to trunk. If you do not have commit access, someone has to
 commit the change for you (with attribution). It is sufficient to add
 a comment to the approved review indicating you cannot commit the patch
 yourself. If you have commit access, there are multiple workflows to commit the