Clarify notification scheme on watched projects

Since 2.1.4 when watching All-Projects became possible
(e7d485ccc478d5a1a1fcdd579251b93c22a8db1f), the presence of a
specific project match would exclude All-Projects rules from being
processed.  This was never clearly documented, although the change
(https://gerrit-review.googlesource.com/15556) and related issue
(Issue 535) discussed why this was the case.

This clarifies the documentation to mention that an All-Projects
watch will be superceded by a more specific project watch, and
changes the behavior such that a more specific project watch will
only prevent a notification at the All-Projects level if the filter
matches.

Change-Id: I7056f692047ccf00f61107cb6ec0d39335131254
2 files changed