js: document what cstring.default should be (nil vs "")

[timotheecour]

arguments for nil:

• this is the current default, so would not be a breaking change
• allows doing things like this:
  refs: Add jsxmlhttprequest nim-lang/fusion#57 (comment)

func open*(this: XMLHttpRequest; metod, url: cstring; async = true; user = cstring.default, password = cstring.default) {.importjs: "#.$1(#, #, #, #, #)".}

which matches https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/open

The optional user name to use for authentication purposes; by default, this is the null value.

arguments for "":

nil is not a valid value for string in js.
This would treat cstring more like a value type instead of a reference type; but note that in js, string is an immutable, so ref vs value doesn't really matter

if we were to change default value to "", this could be done using something similar to nim-lang#16606 (adapting it for string instead of BigInt)

related links

• RFC: jstring #156
• js string should use Uint8Array typed arrays #483
• cstring.default.len raises a js error instead of returning 0 as in other backends nim-lang/Nim#16674
• (different issue) cstring("") != nil fails at CT; isNil sometimes returns false for nil cstring nim-lang/Nim#15730