Bug#743358: say that gencaches might also do nothing

[積丹尼 Dan Jacobson <jidanni@jidanni.org>]

Package: apt
Version: 1.0
Severity: wishlist
File: /usr/share/man/man8/apt-cache.8.gz

Please change
       gencaches
           gencaches creates APT's package cache. This is done implicitly by
           all commands needing this cache if it is missing or outdated.
to say
       gencaches
           gencaches creates APT's package cache if it is missing or
           outdated, else it does nothing. This is done implicitly by
           all commands needing this cache.

(if indeed it is talking about /var/cache/apt/*.bin ?!)