runtime(java): Tidy up the documentation for "ft-java-syntax"

runtime(java): Tidy up the documentation for "ft-java-syntax"

- Reword a few sentences and reformat a few paragraphs.
- Supply absent capitalisation and punctuation.
- Make listed highlighting groups and code stand out.
- Prefix all Java-related global variables with "g:".
- Add spaces around each variable assignment operator.
- Acknowledge that some Javadoc variables are maintained in
  the HTML syntax file.

Also, move the overridable _default_ HTML group links before
the HTML syntax file inclusion in order to implement the
documented diverged settings.

related: #15399

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>