spec: clarify "slice of bytes" and "slice of runes" through examples

The spec section on conversions uses the terms "slice of bytes" and
"slice of runes". While not obviously clear, what is meant are slice
types whose element types are byte or rune types; specifically the
underlying types of the slices' element types must be byte or rune.

Some of this was evident from the examples, but not all of it. Made
this clearer by adding more examples illustrating various permitted
conversions.

Note that the 1.17 compiler did not accept the following conversions:

        string([]myByte{...})
        string([]myRune{...})
        myString([]myByte{...})
        myString([]myRune{...})

(where myByte, myRune, and myString have underlying types of byte,
rune, and string respectively) - it reported an internal error.
But it did accept the inverse conversions:

        []myByte("...")
        []myRune("...")
        []myByte(myString("..."))
        []myRune(myString("..."))

The 1.18 compiler made those conversions symmetric and they are now
permitted in both directions.

The extra examples reflect this reality.

Fixes #23814.

Change-Id: I5a1c200b45ddd0e8c0dc0d11da3a6c39cb2dc848
Reviewed-on: https://go-review.googlesource.com/c/go/+/412094
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 13, 2022",
+	"Subtitle": "Version of June 14, 2022",
 	"Path": "/ref/spec"
 }-->
 
@@ -5245,7 +5245,7 @@ float32(0.49999999)      // 0.5 of type float32
 float64(-1e-1000)        // 0.0 of type float64
 string('x')              // "x" of type string
 string(0x266c)           // "♬" of type string
-MyString("foo" + "bar")  // "foobar" of type MyString
+myString("foo" + "bar")  // "foobar" of type myString
 string([]byte{'a'})      // not a constant: []byte{'a'} is not a constant
 (*int)(nil)              // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
 int(1.2)                 // illegal: 1.2 cannot be represented as an int
@@ -5428,8 +5428,9 @@ the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
 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"
+
+type myString string
+myString(0x65e5)  // "\u65e5" == "日" == "\xe6\x97\xa5"
 </pre>
 </li>
 
@@ -5442,8 +5443,12 @@ string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'})   // "hellø"
 string([]byte{})                                     // ""
 string([]byte(nil))                                  // ""
 
-type MyBytes []byte
-string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'})  // "hellø"
+type bytes []byte
+string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'})    // "hellø"
+
+type myByte byte
+string([]myByte{'w', 'o', 'r', 'l', 'd', '!'})       // "world!"
+myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'})   // "🌍"
 </pre>
 </li>
 
@@ -5457,8 +5462,12 @@ string([]rune{0x767d, 0x9d6c, 0x7fd4})   // "\u767d\u9d6c\u7fd4" == "白鵬翔"
 string([]rune{})                         // ""
 string([]rune(nil))                      // ""
 
-type MyRunes []rune
-string(MyRunes{0x767d, 0x9d6c, 0x7fd4})  // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+type runes []rune
+string(runes{0x767d, 0x9d6c, 0x7fd4})    // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+
+type myRune rune
+string([]myRune{0x266b, 0x266c})         // "\u266b\u266c" == "♫♬"
+myString([]myRune{0x1F30E})              // "\U0001f30e" == "🌎"
 </pre>
 </li>
 
@@ -5470,7 +5479,10 @@ yields a slice whose successive elements are the bytes of the string.
 []byte("hellø")             // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
 []byte("")                  // []byte{}
 
-MyBytes("hellø")  // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+bytes("hellø")              // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+
+[]myByte("world!")          // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
+[]myByte(myString("🌏"))    // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
 </pre>
 </li>
 
@@ -5479,10 +5491,13 @@ Converting a value of a string type to a slice of runes type
 yields a slice containing the individual Unicode code points of the string.
 
 <pre>
-[]rune(MyString("白鵬翔"))  // []rune{0x767d, 0x9d6c, 0x7fd4}
+[]rune(myString("白鵬翔"))   // []rune{0x767d, 0x9d6c, 0x7fd4}
 []rune("")                  // []rune{}
 
-MyRunes("白鵬翔")           // []rune{0x767d, 0x9d6c, 0x7fd4}
+runes("白鵬翔")              // []rune{0x767d, 0x9d6c, 0x7fd4}
+
+[]myRune("♫♬")              // []myRune{0x266b, 0x266c}
+[]myRune(myString("🌐"))    // []myRune{0x1f310}
 </pre>
 </li>
 </ol>