be more explicit about initialization of embedded fields.

R=rsc
CC=golang-dev
https://golang.org/cl/186161

doc/effective_go.html

@@ -1395,7 +1395,7 @@ func (b ByteSize) String() string {
 </pre>
 <p>
 The expression <code>YB</code> prints as <code>1.00YB</code>,
-while <code>ByteSize(1e13)</code> prints as <code>9.09TB</code>,
+while <code>ByteSize(1e13)</code> prints as <code>9.09TB</code>.
 </p>
 
 <h3 id="variables">Variables</h3>
@@ -1878,12 +1878,15 @@ but does not give them field names.
 // ReadWriter stores pointers to a Reader and a Writer.
 // It implements io.ReadWriter.
 type ReadWriter struct {
-    *Reader
-    *Writer
+    *Reader  // *bufio.Reader
+    *Writer  // *bufio.Writer
 }
 </pre>
 <p>
-This struct could be written as
+The embedded elements are pointers to structs and of course
+must be initialized to point to valid structs before they
+can be used.
+The <code>ReadWriter</code> struct could be written as
 </p>
 <pre>
 type ReadWriter struct {
@@ -1933,15 +1936,16 @@ type Job struct {
 The <code>Job</code> type now has the <code>Log</code>, <code>Logf</code>
 and other
 methods of <code>log.Logger</code>.  We could have given the <code>Logger</code>
-a field name, of course, but it's not necessary to do so.  And now we can
-log to a <code>Job</code>:
+a field name, of course, but it's not necessary to do so.  And now, once
+initialized, we can
+log to the <code>Job</code>:
 </p>
 <pre>
 job.Log("starting now...")
 </pre>
 <p>
 The <code>Logger</code> is a regular field of the struct and we can initialize
-it in the usual way.
+it in the usual way with a constructor,
 </p>
 <pre>
 func NewJob(command string, logger *log.Logger) *Job {
@@ -1949,6 +1953,12 @@ func NewJob(command string, logger *log.Logger) *Job {
 }
 </pre>
 <p>
+or with a composite literal,
+</p>
+<pre>
+job := &amp;Job{command, log.New(os.Stderr, nil, "Job: ", log.Ldate)}
+</pre>
+<p>
 If we need to refer to an embedded field directly, the type name of the field,
 ignoring the package qualifier, serves as a field name.  If we needed to access the
 <code>*log.Logger</code> of a <code>Job</code> variable <code>job</code>,