Rename the ".general" rules

[ehuss]

The "general" suffix seemed too generic to me, and also didn't quite fit with some of the style in the rest of the grammar.

Some of these are difficult to come up with good names for, and I'm not entirely happy with this. My thinking is:

• .intro is for informative introductions to a section. My intention is that every normative rule is testable. However, many of these .intro rules define terms, and I think those should possibly be normative (like ISO/IEC Directives).

  I would like to rethink how .intro works. I appreciate having introductions that are more generalized, but it is often very difficult to introduce a section without defining some terms first.

  I would also like to come up with a better strategy of "this is a testable rule" versus "this rule defines a term, but that by itself isn't testable".

• Switched some of these to be more specific words tailored to their context.

• Used .def for some of these that define a thing. .def is used in a variety of other places. We have not standardized on it, and I'm not sure it is working in a way that is clear (is it defining a term? behavior? rule? general principle?).

  I think this is being used in ways that are inconsistent, and would like to figure out something better.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.