Cleaned up default descriptions in reflection.jl (#23543)
* Cleaned up default descriptions in reflection.jl
Redundant default descriptions were removed. In addtion, square brackets to denote optional arguments were replaced by default syntax:
code_native([io], f, types, [syntax]) -> code_native(io=STDOUT, f, types, syntax=att)
* Use `[io=STDOUT,]` to show default argument