add 'unnumbered' style to footnote parts

The function bound to `make-my-notes` by `define-footnote` (as in:

```
  (define-footnote my-note make-my-notes)
```

) now produces a part with the `unnumbered` style.

This way, footnote sections can go between any two sections on a page
 without upsetting the section numbers.

scribble-doc/scriblib/scribblings/footnote.scrbl

@@ -14,13 +14,12 @@ Creates a margin note for HTML and a footnote for Latex/PDF output.}
 
 @defform[(define-footnote footnote-id footnote-part-id)]{
 
-Binds @racket[footnote-id] to a form like @racket[note] that generates
-a footnote in HTML output as well as Latex/PDF output. To trigger the
-HTML output of the footnotes that are registered through
-@racket[footnote-id], the function bound to @racket[footnote-part-id]
-must be called at a position that corresponds the bottom of the HTML
-page. (The generated section will not show a title or appear in a
-table of contents; it will look like a footnote area.)
+Binds @racket[footnote-id] to a form like @racket[note] that registers a
+footnote.
+Binds @racket[footnote-part-id] to a function that generates a section to
+display the registered footnotes.
+(The section generated by @racket[footnote-part-id] will not show a title or
+appear in a table of contents; it will look like a footnote area.)
 
 Beware that any content passed to @racket[footnote-id] will occur
 twice in at least an intermediate form of the document, and perhaps
@@ -28,6 +27,24 @@ also in the rendered form of the document. Consequently, the content
 passed to @racket[footnote-id] should not bind link targets or include
 other one-time declarations.}
 
+Example:
+@codeblock|{
+  #lang scribble/manual
+  @require[scriblib/footnote]
 
+  @define-footnote[my-note make-my-note]
 
+  @title{Months of the Year}
 
+  @section{January}
+  January has 31 days.
+
+  @section{February}
+  February has 28 days in common years.@my-note{In leap years,
+  February has 29 days.}
+
+  @make-my-note[]
+
+  @section{March}
+  March has 30 days.
+}|

scribble-lib/scriblib/footnote.rkt

@@ -71,7 +71,7 @@
    #f
    (list `(part ,(generated-tag)))
    #f
-   (make-style #f '(hidden toc-hidden))
+   (make-style #f '(unnumbered hidden toc-hidden))
    null
    (list
     (make-traverse-block