tools/translate.sh: Improve usage
- Update .po file with the source document before translating.
- Allow translation of a specific document.
Translate all es documents: ./tools/translate.sh documentation es
Translate only pt-br articles/bsdl-gpl: ./tools/translate.sh documentation pt-br articles/bsdl-gpl
Reviewed by: fernape
Differential Revision: https://reviews.freebsd.org/D33067