spec: clarify when range expression is evaluated

Clarify that the range expression of a "for" loop is called *just* once to
rule out that it might be re-evaluated after each iteration.

Change-Id: Iedb61cd29e5238ac0168b8ac01c34d6208cc4312
Reviewed-on: https://go-review.googlesource.com/c/go/+/582775
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: dolmen

doc/go_spec.html

@@ -6643,7 +6643,7 @@ <h4 id="For_range">For statements with <code>range</code> clause</h4>
 </p>
 
 <p>
-The range expression <code>x</code> is evaluated once before beginning the loop,
+The range expression <code>x</code> is evaluated before beginning the loop,
 with one exception: if at most one iteration variable is present and <code>x</code> or
 <a href="#Length_and_capacity"><code>len(x)</code></a> is <a href="#Constants">constant</a>,
 the range expression is not evaluated.