[rygel] Add a comment about the docs build for the future.

[Murray Cumming <murrayc src gnome org>]

commit d8e9174ebadd0393d464244a5e2cea80f3609538
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Dec 11 10:38:04 2012 +0100

    Add a comment about the docs build for the future.

 doc/reference/doc-build.am |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/doc/reference/doc-build.am b/doc/reference/doc-build.am
index 85defa2..1dac20f 100644
--- a/doc/reference/doc-build.am
+++ b/doc/reference/doc-build.am
@@ -26,6 +26,12 @@ valadoc-gen: $(VALA_SOURCE_FILES_WITH_PATHS)
 # Avoid regeneration of the html if none of its dependencies have changed:
 gtkdoc/html/index.html: gtkdoc-gen
 
+# The use of extra .xml DocBook files, just for the gtk-doc output
+# (in RYGEL_GTKDOC_DOCS_XML_EXTRA),
+# will be unnecessary when valadoc has some way to add extra pages of
+# documentation. See https://bugzilla.gnome.org/show_bug.cgi?id=680798
+# The whole gtk-doc build might then get much simpler like it once was:
+# http://git.gnome.org/browse/rygel/commit/doc/reference/Makefile.am?id=80b68029d42fec3fbbb9c0dbdda99c065bddcb0f
 RYGEL_GTKDOC_DOCS_XML = \
 	$(top_srcdir)/doc/reference/lib$(RYGEL_DOCBUILD_MODULE)/gtkdoc/lib$(RYGEL_DOCBUILD_MODULE)-docs.xml \
 	$(top_srcdir)/doc/reference/lib$(RYGEL_DOCBUILD_MODULE)/gtkdoc/overview.xml $(RYGEL_GTKDOC_DOCS_XML_EXTRA)