Rework String UTF-8 Documentation

**This Commit**
Adds some clarity around indexing into Strings and the constraints
driving various decisions there.

**Why?**
The [`String` documentation][0] mentions how `String`s can't be indexed
but `Range` has an implementation for `SliceIndex<str>`. This can be
confusing. There are also several statements to explain the lack of
`String` indexing:

- the inability to index into a `String` is an implication of UTF-8
  encoding
- indexing into a `String` could not be constant-time with UTF-8
  encoding
- indexing into a `String` does not have an obvious return type

This last statement made sense but the first two seemed contradictory to
the documentation around [`SliceIndex<str>`][1] which mention:

- one can index into a `String` with a `Range` (also called substring
  slicing but it uses the same syntax and the method name is `index`)
- `Range` indexing into a `String` is constant-time

To resolve this seeming contradiction the documentation is reworked to
more clearly explain what factors drive the decision to disallow
indexing into a `String` with a single number.

[0]: https://doc.rust-lang.org/stable/std/string/struct.String.html#utf-8
[1]: https://doc.rust-lang.org/stable/std/slice/trait.SliceIndex.html#impl-SliceIndex%3Cstr%3E

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