Document that a derived archive must call init() itself

Since Boost 1.73, the CRTP base archive classes no longer call `init()`
from their constructors: doing so downcast this to the most derived
class while it was still being constructed, which is undefined behavior
and tripped the sanitizers. The derivation guide still showed the old
pattern, in which a class derived from one of the `xxx_oarchive_impl`
templates relied on the base to write the archive header. Following it
now yields an archive that never writes its header.

The guide now states that the most derived class is responsible for
calling `init()` in its own constructor body, honoring `no_header`, and
shows how, noting that the log_archive example is empty only because it
suppresses the header.

This also corrects the curiously recurring template argument in the
further-derivation sketch, which named `xml_oarchive` instead of
`log_archive`.

Refs issue #182.

doc/derivation.html

@@ -70,8 +70,28 @@ class log_archive :
     <li><i>Note the</i> <code style="white-space: normal">log_archive</code> <i>between the</i> &lt;&gt;
 This is required so that base classes can downcast their <code style="white-space: normal">this</code> pointer
 to the most derived class.  This is referred to as <b>C</b>uriously <b>R</b>ecurring
-<b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>.  
+<b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>.
 It is used to implement static polymorphism.
+    <li><i>The most derived class is responsible for calling</i> <code style="white-space: normal">init()</code>.
+The archive header (signature and version) is written by <code style="white-space: normal">init()</code>.
+Before Boost 1.73, the <b>CRTP</b> base called <code style="white-space: normal">init()</code> from its own
+constructor, but that downcast <code style="white-space: normal">this</code> to the most derived class
+while that class was still being constructed, which is undefined behavior.  As of 1.73, the base
+classes no longer do this, so a derived archive that wants the standard header must call
+<code style="white-space: normal">init()</code> itself, in its own constructor body, after the base
+sub-objects have been constructed, honoring the <code style="white-space: normal">no_header</code> flag:
+<pre><code>
+    log_archive(std::ostream &amp; os, unsigned int flags = 0) :
+        xml_oarchive_impl&lt;log_archive&gt;(os, flags)
+    {
+        if(0 == (flags &amp; boost::archive::no_header))
+            init();
+    }
+</code></pre>
+The <code style="white-space: normal">log_archive</code> shown in this example passes
+<code style="white-space: normal">no_header</code>, so it has no header to write and its constructor
+body is empty; see <code style="white-space: normal">xml_oarchive</code>, or the
+<code style="white-space: normal">portable_binary_oarchive</code> example, for archives that do write one.
     <li><i>Base classes need to be explicitly given access to the derived class.</i>
 This can be done by making members public or by including friend declarations for
 the base classes.
@@ -135,7 +155,7 @@ class log_archive :
 {
 public:
     log_archive(std::ostream &amp; os, unsigned int flags = 0) :
-        log_archive_impl&lt;xml_oarchive&gt;(os, flags)
+        log_archive_impl&lt;log_archive&gt;(os, flags)
     {}
 };
 </code></pre>