add docs for creatures search overlay

docs/plugins/sort.rst

@@ -91,3 +91,11 @@ and "ranged combat potential" are explained in detail here:
 https://www.reddit.com/r/dwarffortress/comments/163kczo/enhancing_military_candidate_selection_part_3/
 "Mental stability" is explained here:
 https://www.reddit.com/r/dwarffortress/comments/1617s11/enhancing_military_candidate_selection_part_2/
+
+Creatures overlay
+-----------------
+
+The search widget that appears on the "Creatures" info panel sub-tabs (e.g.
+"Citizens", "Pets", etc.) can search the lists by name and other shown
+attibutes. For example, searching for ``caged`` will show all caged prisoners
+on the "Other" tab.

plugins/lua/sort/creatures.lua

@@ -145,14 +145,14 @@ end
 local function overall_training_search(data, filter, incremental)
     general_search(creatures.atk_index, function(elem)
             local raw = df.creature_raw.find(elem)
-            if not raw then return '' end
+            if not raw then return end
             return raw.name[1]
         end, nil, data, filter, incremental)
 end
 
 local function assign_trainer_search(data, filter, incremental)
     general_search(creatures.trainer, function(elem)
-            if not elem then return nil end
+            if not elem then return end
             return ('%s %s'):format(dfhack.TranslateName(elem.name), dfhack.units.getProfessionName(elem))
         end, nil, data, filter, incremental)
 end