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
Fix note about retaining buffer contents
Change File.readln(buf) example not to idup each line

Now lines starting with '#' are ignored.
Lines starting with 'q' no longer end the program.
Simplify File.readln(buf) example using a single output string

This is probably more performant than an array of idup'ped strings.
(Suggested by Steven Schveighoffer).
Merge pull request #1380 from ntrel/stdio-doc-fixes

Fix & improve readln, File.readln docs