Docs: Make our AsciiDoc markup more compatible with AsciiDoctor.

Docs: Make our AsciiDoc markup more compatible with AsciiDoctor.

Start using markup that is preferred by Asciidoctor but compatible with
both generators.

Add a missing "cpp" attribute and set a couple of Asciidoctor-specific
compatibility attributes.

Change-Id: Iff4c31362e4493b97a85f46db2c39b18c336536f
Reviewed-on: https://code.wireshark.org/review/25600
Reviewed-by: Gerald Combs <gerald@wireshark.org>