Fix & improve readln, File.readln docs

* Purge phantom `fp` parameter.
* Add missing parameters and throws info.
* Fix `File.readln(buf)` example to use `char[] buf` for reuse, not `string`.
* Fix swapped `readln()` and `File.readln()` examples.
* Separate `readln()` and `readln(buf)` docs because the behaviour and return types are different.