nixos/libinput: Clarify that accelSpeed does not apply to custom profile

Add note to the description of the accelSpeed option to make it clear
that is only applies when flat or adaptive profile is in use.

nixos/modules/services/hardware/libinput.nix

@@ -40,7 +40,10 @@ let cfg = config.services.libinput;
         type = types.nullOr types.str;
         default = null;
         example = "-0.5";
-        description = "Cursor acceleration (how fast speed increases from minSpeed to maxSpeed).";
+        description = ''
+            Cursor acceleration (how fast speed increases from minSpeed to maxSpeed).
+            This only applies to the flat or adaptive profile.
+          '';
       };
 
       accelPointsFallback = mkOption {