beginner error: do not use $a and $b outside of sort

[szabgab]

If someone writes my $a or my $b (as could have be seen in trunk/Padre/lib/Padre/Document/Perl.pm until r8595) it should be pointed out that it is a bad practice to use either of those as they are special variables of sort().

Also any other use that is clearly not related to sort should be discouraged. I think and assignment to $a or $b should be also caught
but make sure to allow for external sort functions eg.

sub sort_by_num { $a <=> $b }

is a legitimate code.

Such practices should be probably caught in comments as well to avoid distributing bad examples?

[zenogantner]

fix typo in summary