process with undef and empty list clarified

An empty list just 'return's, an explicit 'undef' always gives
'undef'.
Better documentation for Text::Filter usage

Explains the recommended settings better, and the potential for
lines containing no characters ('') appearing in the output.
Perldoc cleanup

- Removed all mentions of "selector methods"
- Removed whitespace at the end of lines.
Added script 'text-lossy' to distribution

This causes 'make' to install the script 'text-lossy' as well.