manual: add known issues entry about XOAUTH2 "invalid_grant"

- Split the manual asciidoc source file.
- Improve the Makefile to build the manuals.

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/374
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

docs/Makefile

@@ -18,11 +18,11 @@ $(HTML_TARGETS): %.html : %.rst
 
 man: offlineimap.1 offlineimapui.7
 
-offlineimap.1: offlineimap.txt
-	a2x -v -f manpage $?
+offlineimap.1: offlineimap.txt offlineimap.known_issues.txt
+	a2x -v -d manpage -f manpage $<
 
 offlineimapui.7: offlineimapui.txt
-	a2x -v -f manpage $?
+	a2x -v -d manpage -f manpage $<
 
 api:
 	$(SPHINXBUILD) -b html -d html/doctrees doc-src html