[docs] Recommend std.ascii.newline for portable line terminator in std.stdio

Add note for `readln(), File.readln()` buffer overloads and `byLine(), byLineCopy()`.

Also document `lines` struct constructor.

Note that `lines()` and `readln(), File.readln()` non-buffer overloads currently cannot take a string as terminator, so we can't recommend `std.ascii.newline` there.