display attr-path only when queried available

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

@@ -1066,7 +1066,8 @@ user environment elements, etc. -->
     the derivation, which can be used to unambiguously select it using
     the <link linkend="opt-attr"><option>--attr</option> option</link>
     available in commands that install derivations like
-    <literal>nix-env --install</literal>.</para></listitem>
+    <literal>nix-env --install</literal>. This option only works
+    together with <option>--available</option></para></listitem>
 
   </varlistentry>
 

src/nix-env/nix-env.cc

@@ -914,6 +914,8 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
             throw UsageError(format("unknown flag '%1%'") % arg);
     }
 
+    if (printAttrPath && source != sAvailable)
+        throw UsageError("--attr-path(-P) only works with --available");
 
     /* Obtain derivation information from the specified source. */
     DrvInfos availElems, installedElems;