IPC::Open2 and IPC::Open3 documentation updates

IPC::Open2 and IPC::Open3 documentation updates

* Remove direct usage of bareword filehandles and use lexical filehandles for the first example in each synopsis.
* Add examples using STDIN and STDOUT filehandles, and examples using already open filehandles, with explicit examples of where these handles come from.
* Declare variables with 'my' where appropriate and condense declarations inline.
* Add comments in synopsis describing the purpose of each example.
* Consistency of referencing synopsis variables from the description.
* Replace ambiguous phrase 'null string' with 'empty string or undefined' which is also more correct here.
* Add links to referenced CPAN modules and manpages.
* Better describe the reason for using gensym in IPC::Open3 and how to use it.