update user guide

- Update some project pagelinks.
- Update some of the usage synopses.
- Use <![CDATA[]]> for some the XML examples.
- Tighten up the wording of the namespace section, and add explanation
  about automatic binding.
Revert "avoid false positives for DEFAULT_NS_FAIL_MESSAGE"

This reverts commit 18589bf9ccce2243486cfa91c65710cb944fa841.

Conflicts:
	src/xml_edit.c
	src/xml_select.c
Revert "give a helpful message if doc has default namespace and nothing matched"

This reverts commit 5cb1977bb3127443415a9952b0afad5be9fb54b7.

Conflicts:
	src/xml_edit.c
get rid of "helpful" message about namespaces

It was a mistake to introduce a message like that, there are always
going to be some false positives, which is simply unacceptable in a
scripting tool.