Document how to write multiline regex with comments

It is possible to add multiline and docummented regex to the
tempest_test_regex but it might not be evident at first, although very
useful for some scenarios.

This patch documents how this should be achieved to make it easier for
people in the future.

Change-Id: Id54679debedb1667cc70004b9a28fba921e50c0b

roles/run-tempest/README.rst

@@ -16,9 +16,19 @@ Run Tempest
    :default: ''
 
    A regular expression used to select the tests.
+
    It works only when used with some specific tox environments
    ('all', 'all-plugin'.)
 
+   Multi-line and commented regexs can be achieved by doing this:
+
+       ::
+           vars:
+             tempest_test_regex: |
+               (?x)    # Ignore comments and whitespaces
+               # Line with only a comment.
+               (tempest\.(api|scenario|thirdparty)).*$    # Run only api scenario and third party
+
 .. zuul:rolevar:: tox_venvlist
    :default: smoke