document default(T)

[timotheecour]

this PR explains the subtle difference between default(T) and var a: T.

this also explains why default is a magic and can't (at least not currently) be implemented via template default*[T](t: typedesc[T]): T = (var v: T; v)

note also that even without the range constraint issues, template default*[T](t: typedesc[T]): T = (var v: T; v) would run into another problem (typedesc at CT) exaplained in #7596, ie would require this:

proc defaultImpl[T]():T=
  var a: T
  return a
template default*[T](t: typedesc[T]): T = defaultImpl[T]()

but I didn't mention this in the doc comment.

I explicitly mentioned "curently, it generates binary 0", which leaves open possibility that future nim may assign default(T) differently, eg via:

• User-defined implicit initialization hooks RFCs#252 (comment) (object field static initializers)
• =default type bound operator to override default and var initialization RFCs#290 (=default type bound operator)

[juancarlospaco]

somePointer.default returns ?, someOption.default returns?, nil, error, something else ?.

[timotheecour]

@juancarlospaco I thought these cases were clear from the docs but feel free to do a followup PR to add an example with 1 or 2 more examples, eg doAssert cstring.default == nil