Initial docs for StringRuneEnumerator and SpanRuneEnumerator

[GrabYourPitchforks]

Summary

Adds documentation for StringRuneEnumerator and SpanRuneEnumerator, which are enumerators returned by the String.EnumerateRunes and MemoryExtensions.EnumerateRunes methods. Developers should not use the enumerator types directly and should instead use syntax like the below:

foreach (Rune rune in myString.EnumerateRunes() /* or mySpan.EnumerateRunes() */)
{
    // use 'rune' here
}

The proposed text for the documentation is based somewhat on the existing text for the Span<T>+Enumerator type.

/cc @mairaw @rpetrusha @carlossanlop

[carlossanlop]

Thanks for your PR. I left some comments for you to consider.

[mairaw]

Looks good @GrabYourPitchforks! Left a few comments

[GrabYourPitchforks]

Apologies for the noise - going through and applying suggestions, which will introduce a bunch of tiny commits to the PR.

[mairaw]

If you go to the Files changed tab, you can batch them in a single commit @GrabYourPitchforks 😉

[mairaw]

LGTM. I've added the boilerplate remarks that we add for EIIs as well (I've recently added to the wiki published at https://github.com/dotnet/dotnet-api-docs/wiki/Remarks#explicit-interface-implementation)

[carlossanlop]

@mairaw the build passed, so I think this is good to merge.

[carlossanlop]

@rpetrusha this one's good to merge, the build passed.