Fix some formatting issues

[JeromeMartinez]

As underscores have a specific meaning in Markdown, not escaping them has some ugly results when converted to HTML, e.g.

I tried to catch all fields with underscores, with this rule in mind:

• paragraph titles and table column names have the underscore escaped, not considered as "code", even if only 1 underscore (currently not caught as italic, but I prefer to take no risk for the future in case we change of Markdown processor and this make all titles coherent with always the underscore escaped).
• when used as bitstream fields directly (alone or in a formula), they are always within 2 backquotes (which disables the italic meaning of the undesrcore).

Let me know if you prefer all of the fields within 2 backquotes i.e. also in paragraph titles and table column names.

[dericed]

Can you confirm that the cross-sectional links work after this. For instance in section 3.8.1.6 there should be a link to the coder_type section (4.1.3). Does it still work in the txt and html outputs?

[JeromeMartinez]

bump ;-)