specification of []int(string) and []byte(string).

also clarify other string conversions.

R=rsc, iant, gri, ken2
CC=golang-dev
https://golang.org/cl/207103

doc/go_spec.html

@@ -3227,33 +3227,60 @@ represent the value the conversion succeeds but the result value is
 implementation-dependent.
 </p>
 
-<h4>Conversions to a string type</h4>
+<h4>Conversions to and from a string type</h4>
+
 <ol>
 <li>
-Converting an integer value yields a string containing the UTF-8
+Converting a signed or unsigned integer value to a string type yields a
-representation of the integer.
+string containing the UTF-8 representation of the integer.
+Negative values are converted to <code>"\uFFFD"</code>.
 
 <pre>
-string(0x65e5)  // "\u65e5" == "日" == "\xe6\x97\xa5"
+string('a')  // "a"
+string(-1)  // "\ufffd" == "\xef\xbf\xbd "
+string(0xf8)  // "\u00f8" == "ø" == "\xc3\xb8"
+type MyString string
+MyString(0x65e5)  // "\u65e5" == "日" == "\xe6\x97\xa5"
 </pre>
 </li>
 
 <li>
-Converting a slice of integers yields a string that is the
+Converting a value of type <code>[]byte</code> (or
-concatenation of the individual integers converted to strings.
+the equivalent <code>[]uint8</code>) to a string type yields a
-If the slice value is <code>nil</code>, the result is the empty string.
+string whose successive bytes are the elements of the slice.  If
+the slice value is <code>nil</code>, the result is the empty string.
+
+<pre>
+string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'})  // "hellø"
+</pre>
+</li>
+
+<li>
+Converting a value of type <code>[]int</code> to a string type yields
+a string that is the concatenation of the individual integers
+converted to strings.  If the slice value is <code>nil</code>, the
+result is the empty string.
 <pre>
 string([]int{0x767d, 0x9d6c, 0x7fd4})  // "\u767d\u9d6c\u7fd4" == "白鵬翔"
 </pre>
 </li>
 
 <li>
-Converting a slice of bytes yields a string whose successive
+Converting a value of a string type to <code>[]byte</code> (or <code>[]uint8</code>)
-bytes are those of the slice. If the slice value is <code>nil</code>,
+yields a slice whose successive elements are the bytes of the string.
-the result is the empty string.
+If the string is empty, the result is <code>[]byte(nil)</code>.
 
 <pre>
-string([]byte{'h', 'e', 'l', 'l', 'o'})  // "hello"
+[]byte("hellø")  // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+</pre>
+</li>
+
+<li>
+Converting a value of a string type to <code>[]int</code> yields a
+slice containing the individual Unicode code points of the string.
+If the string is empty, the result is <code>[]int(nil)</code>.
+<pre>
+[]int(MyString("白鵬翔"))  // []int{0x767d, 0x9d6c, 0x7fd4}
 </pre>
 </li>
 </ol>
@@ -3847,7 +3874,7 @@ statement</a>.  Function calls on the left hand side will be evaluated
 exactly once per iteration.
 </p>
 <p>
-For strings, the "range" clause iterates over the Unicode code points
+For a value of a string type, the "range" clause iterates over the Unicode code points
 in the string.  On successive iterations, the index variable will be the
 index of the first byte of successive UTF-8-encoded code points in the string, and
 the second variable, of type <code>int</code>, will be the value of
@@ -4777,5 +4804,6 @@ The following minimal alignment properties are guaranteed:
 <ul>
 	<li><span class="alert">Implementation does not honor the restriction on goto statements and targets (no intervening declarations).</span></li>
 	<li><span class="alert">Method expressions are not implemented.</span></li>
+	<li><span class="alert">Conversions from strings to <code>[]int</code> and <code>[]byte</code> are not implemented..</span></li>
 	<li><span class="alert">Gccgo allows only one init() function per source file.</span></li>
 </ul>