[texdoc] changes in aliasing and scoring

[Karl Berry]

    intend to drop regex mode sooner or later since it makes too many

Regex mode still seems useful to me when someone is unsure of the
package name -- but a "substring" mode would probably be just as good as
full regexes.  E.g., I know there's some package about algorithms but I
can't remember the name, so I could run texdoc --mode=substr algo or
some such.

Just my $0.02.