doc/faq: clarify that concurrent maps reads are safe

Fixes #23480

Change-Id: I33b4bdc60b9983ab62d87bfa0ae3ff33304269c0
Reviewed-on: https://go-review.googlesource.com/88515Reviewed-by: Rob Pike <r@golang.org>

doc/go_faq.html

@@ -479,6 +479,15 @@ as when hosting an untrusted program, the implementation could interlock
 map access.
 </p>
 
+<p>
+Map access is unsafe only when updates are occurring.
+As long as all goroutines are only reading—looking up elements in the map,
+including iterating through it using a
+<code>for</code> <code>range</code> loop—and not changing the map
+by assigning to elements or doing deletions,
+it is safe for them to access the map concurrently without synchronization.
+</p>
+
 <h3 id="language_changes">
 Will you accept my language change?</h3>