Add documentation for coverage reports with meson

Hi all,

I have mentioned on a different thread of -docs that we have no
documentation to achieve $subject, so attached is a patch to add
something. This can be done with the following steps:
meson setup -Db_coverage=true .. blah
ninja
meson test
ninja coverage-html

As far as I can see, there is no option to generate anything else than
a HTML report? This portion is telling the contrary, still it does
not seem to work here and ninja does the job with coverage-html or
coverage as only available targets:
https://mesonbuild.com/howtox.html#producing-a-coverage-report

Side issue: the current code generates no reports for the files that
are automatically generated in src/backend/nodes/, which are actually
part of src/include/ for a meson build. I have not looked into that
yet.

Thoughts?
--
Michael