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.
Use 'void main' instead of 'int main' in examples
Change 'is reused' to 'you can reuse' for clarity