Added the missing xml-stylesheet and Xinclude namespace
declarations in pjsip_config.xml and pjsip_manager.xml.

Updated make_xml_documentation to show detailed errors when
xmlstarlet is the validator.  It's now run once with the '-q'
option to suppress harmless/expected messages and if it actually
fails, it's run again without '-q' but with '-e' to show
the actual errors.

Change-Id: I4bdc9d2ea6741e8d2e5eb82df60c68ccc59e1f5e