We have replaced use of `buildbot` with GitLab CI.  Update the
relevant section of our review process documentation.

GitLab now uses a `/-/` component between the `group/project` part of
the URL and the `{issues,merge_requests,tree}` part so that it can
support `group/subgroup/project` with arbitrary depth.

This was accidentally left out of commit 49f5b6f7 (Help: Document the
expire and external discussion resolve states, 2019-05-26,
v3.15.0-rc1~35^2).

Our practice of closing MRs temporarily while discussion
takes place in a separate issue isn't always well understood
by MR authors. Expiring a MR seems to be better understood,
but making it clear that it is also a temporary state is helpful.

The logic is that the describe output is readily available using `git
tag --contains` locally. In addition, for a hypothetical commit which
landed in both v3.9.4 and v3.10.1, there is no "better" tag to refer to.
since v3.10.0's relation to such a commit is unclear either way.

Also mention that a `Fixes` trailer is preferred if the mention is just
to indicate a commit which introduced an error rather than writing a
complete sentence about it.

This builder builds CMake using lint tools but does not run the test
suite.  Document it for reviewers to request with `Do: test -i lint`.

Add a dedicated subsection header for its documentation.

Fixes: #16264.