Add notes for UtfString-related APIs

[terrajobst]

/cc @GrabYourPitchforks @migueldeicaza

[itsamelambda]

Rune is not a precise name unless it actually describes a letter of the runic alphabet. Otherwise it becomes an overloaded term.

Ironically, a rune is already defined to be one of the following Unicode letters in the standard: https://www.unicode.org/charts/PDF/U16A0.pdf

Why not rename UnicodeScalar to Hangul? It would be fun to use in Korea.

[benaadams]

Rune was the term used by the creator of Utf-8 when creating Utf-8 https://www.cl.cam.ac.uk/~mgk25/ucs/utf-8-history.txt

He's now works on golang; which is why golang uses rune https://blog.golang.org/strings

The Unicode block is called Runic not Rune, and they are Runic letter(s), not Rune letter(s)

[terrajobst]

Ben is correct, that’s the rationale.

[0xced]

The naming has already been extensively discussed at https://github.com/dotnet/corefx/issues/24093. I have the impression that most people there think that Rune is not a good name but I'm probably biased.

[aidapsibr]

Going to agree with the logic for the name, the creator named it that IN an implementation of a mainstream language. Other than getting his direct opinion on the matter, this makes sense to me.

[tannergooding]

Rune was the term used by the creator of Utf-8 when creating Utf-8 https://www.cl.cam.ac.uk/~mgk25/ucs/utf-8-history.txt

He's now works on golang; which is why golang uses rune https://blog.golang.org/strings

The Unicode block is called Runic not Rune, and they are Runic letter(s), not Rune letter(s)

@benaadams, I don't think any of that really matters. Lots of people use "bad" names for prototypes, etc.

I think that what really matters is that the current/latest Unicode Term Glossary does not use or define "Rune". It defines and talks in terms of Code Point and Code Unit.

Additionally, if you search through the current Unicode Specification (http://www.unicode.org/versions/Unicode11.0.0/UnicodeStandard-11.0.pdf) for the term "rune", there are a total of 37 hits (starting on page 337) and every hit is talking about the Runic Alphabet and the individual characters ("runes") that comprise it.