[BUG]devguide_xmlrpc: needs a rewrite

[duncdrum]

Describe the bug
New issue collecting:
see #246
see #229

Both only partially addressed. The article is big on inaccurate details, and small actionable info.
We should consider in how far this stuff should be here, the funcdocs, or the javadocs. But since b) and c) MIA. This deserves its own ticket.

see #140
see #203

Expected behavior
The article to document the current applicable syntax and array of options.

Context (please always complete the following information):

• eXist-db Version: [e.g. 5.2.0]
• Java Version: [e.g. Java8u121]
• App Version: [e.g. 5.1.0]