Rollup merge of #94794 - mlodato517:mlodato517-clarify-string-indexing-docs, r=Mark-Simulacrum

Clarify indexing into Strings

**This Commit**
Adds some clarity around indexing into Strings.

**Why?**
I was reading through the `Range` documentation and saw an
implementation for `SliceIndex<str>`. I was surprised to see this and
went to read the [`String`][0] documentation and, to me, it seemed to
say (at least) three things:

1. you cannot index into a `String`
2. indexing into a `String` could not be constant-time
3. indexing into a `String` does not have an obvious return type

I absolutely agree with the last point but the first two seemed
contradictory to the documentation around [`SliceIndex<str>`][1]
which mention:

1. you can do substring slicing (which is probably different than
   "indexing" but, because the method is called `index` and I associate
   anything with square brackets with "indexing" it was enough to
   confuse me)
2. substring slicing is constant-time (this may be algorithmic ignorance
   on my part but if `&s[i..i+1]` is O(1) then it seems confusing that
   `&s[i]` _could not possibly_ be O(1))

So I was hoping to clarify a couple things and, hopefully, in this PR
review learn a little more about the nuances here that confused me in
the first place.

[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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