Add missing doc about strictness (#255)

Author: Lysxia

Data/HashMap/Lazy.hs

@@ -102,4 +102,4 @@ import Prelude ()
 --
 -- This module satisfies the following strictness property:
 --
--- * Key arguments are evaluated to WHNF
+-- * Key arguments are evaluated to WHNF.

Data/HashMap/Strict.hs

@@ -96,3 +96,12 @@ module Data.HashMap.Strict
 import Data.HashMap.Strict.Base as HM
 import qualified Data.HashSet.Base as HS
 import Prelude ()
+
+-- $strictness
+--
+-- This module satisfies the following strictness properties:
+--
+-- 1. Key arguments are evaluated to WHNF;
+--
+-- 2. Keys and values are evaluated to WHNF before they are stored in
+--    the map.