Consisent x- Param/Encoding/Header wording (3.1.1 port of 3919)

It was a bit challenging to figure out where to put the wording
about allowing extension parameters when there are multiple
fixed fields tables, each in their own subsection.

For the Parameter Object (the only one with multiple tables in
past releases), it had been after the last table, but that got
further and further away from what felt like the main part of
the Object description.

I thought I had put it consistently after the initial
"Common Fixed Fields" table, but I put it even before that in
one place (which we don't do anywhere), and I forgot to include
it in the Header Object at all.

This change puts it after the Common Fixed Fields table for
all three, which means that for all Objects it is immediately
after the first Fixed Fields table.