only delete things older than current gen and update logic in doc as

well

doc/manual/command-ref/nix-env.xml

@@ -1349,9 +1349,9 @@ special value <literal>old</literal> to delete all non-current
 generations,  a value such as <literal>30d</literal> to delete all
 generations older than the specified number of days (except for the
 generation that was active at that point in time), or a value such as.
-<literal>+5</literal> to delete all but the number of items specified.
-Periodically deleting old generations is important to make garbage
-collection effective.</para>
+<literal>+5</literal> to only keep the specified items older than the
+current generation. Periodically deleting old generations is important
+to make garbage collection effective.</para>
 
 </refsection>
 
@@ -1461,7 +1461,7 @@ error: no generation older than the current (91) exists</screen>
 <refsection condition="manpage"><title>Environment variables</title>
 
 <variablelist>
-  
+
   <varlistentry><term><envar>NIX_PROFILE</envar></term>
 
     <listitem><para>Location of the Nix profile.  Defaults to the
@@ -1475,6 +1475,6 @@ error: no generation older than the current (91) exists</screen>
 </variablelist>
 
 </refsection>
-  
+
 
 </refentry>

src/libstore/profiles.cc

@@ -160,19 +160,24 @@ void deleteGenerations(const Path & profile, const std::set<unsigned int> & gens
 void deleteGenerationsGreaterThan(const Path & profile, int max, bool dryRun)
 {
     PathLocks lock;
-
     lockProfile(lock, profile);
 
     int curGen;
+    bool fromCurGen = false;
     Generations gens = findGenerations(profile, curGen);
-
     for (auto i = gens.rbegin(); i != gens.rend(); ++i) {
-        if (max) {
+        if (i->number == curGen) {
+            fromCurGen = true;
             max--;
             continue;
         }
-        if (i->number != curGen)
+        if (fromCurGen) {
+            if (max) {
+                max--;
+                continue;
+            }
             deleteGeneration2(profile, i->number, dryRun);
+        }
     }
 }