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