UG: Tuning installation instructions

- Moved documentation about entry poits to end.
- General clean-up and tuning.

Related to #1779.
xml atests: fixed typo and argument name
XML: Fix handling elements w/o namespace inside elements w/ ns.

The problem occurred when namespace prefixes in Clarck notation were
stripped (default behavior) and affected both the standard etree and
lxml modes.

After the fix elements w/o ns inside elements w/ ns get `xmlns=""`
when using the standard etree. When using lxml, namespace prefixes are
preserved correctly. These fixes resolve #1784.
Update README.rst