The code base and documentation refer to ``sub`` symbols as:

- subroutines
- functions
- methods
- procedures
- callbacks

This change focuses on always referring to them as subroutines in the
documentation. During this documentation sweep, there were a lot of
false positives since VCL, VMODs and HTTP also share that vocabulary
and I may have missed false negatives, especially for generated bits
of documentation.

Some parts of the documentation were not touched on purpose: upgrade
notes and changelogs of past releases, PHK random outbursts, etc.

Now that the documentation is hopefully taken care of the same needs
to happen in the code base, starting with user-facing error messages
and then data structures names, function names etc.

There will be however one exception. Anything referring to the scope
of a subroutine, but being worded as if it referred to the subroutine
itself will use scope instead.

As this will touch a lot of code, it will be done incrementally, much
like this change focused on non-generated documentation (and VCC files
don't count as generated in this context).

I also fixed RST formatting issues I noticed during my sweep.

Better diff with the --word-diff --ignore-all-space options.