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
1 file changed