xpdf: added long description

[aakropotkin]

The added description will help folks who are looking for a specific tool to be directed here.
When looking for texttopdf it was a mild hassle to need to turn to google rather than nixpkgs to find the proper package.
When someone else had the same hassle just now in the IRC I thought I'd add this to help remedy others who are searching.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[7c6f434c]

Thanks, explicit documentation is always nice.

In future, please include the package names into commit messages. (see CONTRIBUTING.md link in the template)

[aakropotkin]

Thanks, explicit documentation is always nice.

In future, please include the package names into commit messages. (see CONTRIBUTING.md link in the template)

Thanks for the tip, I will start looking at CONTRIBUTING for better commit messages from now on.