> Well, there's specifying a second argument to the FD macro
> which causes the footnote counter to get reset at each
> first-level heading, but that appears to be just as
> undocumented.
>From HP's "Text Formatting User's Guide" (ca. 1991):
If the second argument is specified, automatically-numbered
footnotes begin again with 1 whenever a first-level heading
is encountered. This is most useful with the "section-page"
page numbering scheme. As an example, the input line
.FD "" 1
maintains the default formatting style and causes footnotes
to be numbered afresh after each first-level heading in a
document.