This allows us to skip the expensive man-db profile hook in most cases.
Suggested by Liliana Marie Prikler <liliana.prikler@gmail.com>.
* guix/profiles.scm (manual-database/optional): New procedure.
(%default-profile-hooks): Use it instead of 'manual-database'.
* doc/guix.texi (Documentation): Add footnote about 'man -k' database
creation.