Add impl-trait text from edition-guide, mostly unchanged except for
reformatting to one sentence per line.

Other documentation makes it reasonably clear that `impl Trait` in
argument position allows the _caller_ to specify the type.

Reword a lot of things to sound less like an update and more like a
reference. Add local links in a few places.

Rename and rework the "Using `impl Trait` in more places"
into a "Limitations" section.

Rework "More details" to focus on the differences between `impl Trait`
and generic type parameters in return position. Delete the `.parse()`
examples because they're somewhat verbose for a reference.

Use only the closure example to illustrate the difference between
returning trait objects vs `impl Trait`. Also mention usefulness in
returning iterators.

Be more consistent about using "argument" to mean a concrete value
provided by a function caller, and "parameter" to mean the abstract
input to the function.

Retain a few instances of the phrasing "in argument position",
because it appears in the RFCs and implementation. Also make a note
about the historical terminology inconsistency.