Bug#842230: Say which database, and mention some examples of "frontend methods"

[積丹尼 Dan Jacobson]

Package: dpkg
Version: 1.18.10
Severity: wishlist

    dpkg: warning: package not in database at line 5: sphinx-common:all
    dpkg: warning: found unknown packages; this might mean the available 
database
    is outdated, and needs to be updated through a frontend method


This should say which database, and mention some examples of "frontend methods".