PACKAGERS: Document the optional Doxygen usage

Also add a note that packagers should check the licensing
of the Doxygen output.

(cherry picked from commit 541406bee3)

PACKAGERS

@@ -44,8 +44,8 @@ Information to packagers of XZ Utils
                         lzmadec binary for compatibility with LZMA Utils
         liblzma         liblzma.so.*
         liblzma-devel   liblzma.so, liblzma.a, API headers
-        liblzma-doc     Doxygen-generated liblzma API docs (HTML),
-                        example programs
+        liblzma-doc     Example programs and, if enabled at build time,
+                        Doxygen-generated liblzma API docs (HTML)
 
 
 2. Package description
@@ -170,13 +170,14 @@ Information to packagers of XZ Utils
     can be replaced with a symlink if your distro ships with shared
     copies of the common license texts.
 
-    The Doxygen-generated documentation (HTML) for the liblzma API
-    headers is included in the source release and will be installed by
-    "make install" to $docdir/api. All JavaScript is removed to
-    simplify license compliance and to reduce the install size. If the
-    liblzma API documentation is not desired, either run configure with
-    --disable-doc or remove the doc/api directory before running
-    "make install".
+    The Doxygen-generated liblzma API documentation (HTML) is built and
+    installed if the configure option --enable-doxygen is used (it's
+    disabled by default). This requires that Doxygen is available. The
+    API documentation is installed by "make install" to $docdir/api.
+
+        NOTE: The files generated by Doxygen include content from
+        Doxygen itself. Check the license info before distributing
+        the Doxygen-generated files.
 
 
 6. Extra files