spec: document fp rounding during explicit constant conversion

The gc compilers already behave this way. It was an explicit
decision during the very long constant design discussions,
but it appears not to have made it into the spec.

Fixes #4398.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7313069

doc/go_spec.html

@@ -3470,6 +3470,14 @@ type <code>T</code> in any of these cases:
 	<code>x</code> is representable by a value of type <code>T</code>.
 	</li>
 	<li>
+	<code>x</code> is a floating-point constant,
+	<code>T</code> is a floating-point type,
+	and <code>x</code> is representable by a value
+	of type <code>T</code> after rounding using
+	IEEE 754 round-to-even rules.
+	The constant <code>T(x)</code> is the rounded value.
+	</li>
+	<li>
 	<code>x</code> is an integer constant and <code>T</code> is a
 	<a href="#String_types">string type</a>.
 	The same rule as for non-constant <code>x</code> applies in this case
@@ -3485,6 +3493,7 @@ Converting a constant yields a typed constant as result.
 uint(iota)               // iota value of type uint
 float32(2.718281828)     // 2.718281828 of type float32
 complex128(1)            // 1.0 + 0.0i of type complex128
+float32(0.49999999)      // 0.5 of type float32
 string('x')              // "x" of type string
 string(0x266c)           // "♬" of type string
 MyString("foo" + "bar")  // "foobar" of type MyString