This is just a first pass to specifying how each thing introduces
a name, and where it is introduced. This is missing `use` and `Self`.
(And a few things I don't feel like need elaboration, like loop labels,
but those can be added if desired.)

Since "namespace" has a specific meaning in Rust, I feel like it would
be good not to overload it too much with the sense of a scope container
like a C++ namespace.

This attempts to update the `use` chapter to explain the kinds of paths
and syntax it supports in more detail. This is not an exhaustive
explanation of resolution, and there are several things it does not
cover.

This was changed in rust-lang/rust#56414
to favor in-scope items.

We've merged PR rust-lang#1040, so we can remove the TODO and update the link
to point to the specific section.

We replace some commas with semicolons where that's the right thing to
do grammatically.

Where we have "an X is... they are...", we replace that with "an X
is... Xs are..." for reasons of avoiding a mismatch between the
plurality of the pronoun and its referent.

We replace "implementing type" and "defining type" with "type being
implemented" and "type being defined", since there is in general a
difference (e.g. "the driving force" vs "the force being driven"), and
these seem more like the latter than the former.

There's a place where we had said, "glob imports are allowed to import
conflicting names into the same *namespaces*" (emphasis added).  It
makes sense what this is trying to say by using the plural there.  But
it just reads better to use the singular, and if it's true for the
singular, it's clearly also true to the plural, so we make that
change.

We had mentioned and demonstrated that a name is allowed to be
redundantly imported by multiple glob imports, but we hadn't said
that it is allowed to then be *used*.

Given the example that directly proceeds this, it seems important to
make a note of that, so let's do that, and let's extend the example to
demonstrate this.

The claim here was that:

> Glob imports are allowed to import conflicting names in the same
> namespace as long as the name is not used *or shadowed*.

It's true that the name being used will cause an error.  But it's not
true that the name being shadowed will cause one, so let's remove
that part.

It's a bit more common throughout the Reference, including even within
this PR, to say "For example:" rather than just "Example:", and it's
more grammatically regular, so let's fix up the ones that went the
other way in this branch.

These were removed in rust-lang/rust#103578.