doc: go_mem: clarify Once docs

Fixes #27808

Change-Id: Ia643d51004c47953642a2ba41dfed281f1112be6
Reviewed-on: https://go-review.googlesource.com/c/155637Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go_mem.html

@@ -418,8 +418,12 @@ func twoprint() {
 </pre>
 
 <p>
-calling <code>twoprint</code> causes <code>"hello, world"</code> to be printed twice.
-The first call to <code>doprint</code> runs <code>setup</code> once.
+calling <code>twoprint</code> will call <code>setup</code> exactly
+once.
+The <code>setup</code> function will complete before either call
+of <code>print</code>.
+The result will be that <code>"hello, world"</code> will be printed
+twice.
 </p>
 
 <h2>Incorrect synchronization</h2>