doc: document new hash/maphash package

Update #36878
Update #28322

Change-Id: I793c7c4dbdd23fdecd715500e90b7cc0cbe4cea5
Reviewed-on: https://go-review.googlesource.com/c/go/+/217099
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: randall77

doc/go1.14.html

@@ -432,7 +432,13 @@ <h2 id="library">Core library</h2>
 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash">hash/maphash</a></dt>
   <dd>
     <p><!-- CL 186877 -->
-      TODO: <a href="https://golang.org/cl/186877">https://golang.org/cl/186877</a>: add hashing package for bytes and strings
+      This new package provides hash functions on byte sequences.
+      These hash functions are intended to be used to implement hash tables or
+      other data structures that need to map arbitrary strings or byte
+      sequences to a uniform distribution of integers.
+    </p>
+    <p>
+      The hash functions are collision-resistant but not cryptographically secure.
     </p>
 
 </dl><!-- hash/maphash -->