nix-env: Minor change to '--delete-generations Nd' semantics

The option '--delete-generations Nd' deletes all generations older than N
days. However, most likely the user does not want to delete the
generation that was active N days ago.

For example, say that you have these 3 generations:

1: <30 days ago>
2: <15 days ago>
3: <1 hour ago>

If you do --delete-generations 7d (say, as part of a cron job), most
likely you still want to keep generation 2, i.e. the generation that was
active 7 days ago (and for most of the past 7 days, in fact).

This patch fixes this issue. Note that this also affects
'nix-collect-garbage --delete-older-than Nd'.

Thanks to @roconnor for noticing the issue!

1 files changed, 3 insertions, 2 deletions

diff --git a/doc/manual/nix-collect-garbage.xml b/doc/manual/nix-collect-garbage.xml
index a97e3b7c6b..cf870740f4 100644
--- a/doc/manual/nix-collect-garbage.xml
+++ b/doc/manual/nix-collect-garbage.xml
@@ -45,8 +45,9 @@ which deletes all old generations of all profiles in
 impossible); and
 <option>--delete-older-than</option> <replaceable>period</replaceable>,
 where period is a value such as <literal>30d</literal>, which deletes
-all non-current generations that are older than the specified number of
-days in all profiles in <filename>/nix/var/nix/profiles</filename>.
+all generations older than the specified number of days in all profiles
+in <filename>/nix/var/nix/profiles</filename> (except for the generations
+that were active at that point in time).
 </para>
 
 </refsection>