Update fn:parse-html to apply review feedback.

[rhdunn]

This PR applies the following review comments:

• QT4CG-016-03: RD to add a note clarifying “known character encoding”
• QT4CG-016-04: RD to add a note clarifying the “”/”” html/version combination
• QT4CG-016-05: RD to add a “todo” noting the dependency on keyword arguments
• QT4CG-016-06: RD to reword the introduction to mapping to clarify who’s doing the mapping
• QT4CG-016-08: RD to clarify how namespace comparisons are performed.
• QT4CG-016-09: RD to add a note stating that the local name should always be lowercase
• QT4CG-016-10: RD to consider how to clarify parsed entity parsing.