If you want to document one of the epiphany files you can do it
by the following steps:

- Add documentation headers to the .c file public functions

For example:

/**
 * ephy_session_autoresume:
 * @session: a #EphySession
 *
 * Resume a crashed session when necessary (interactive)
 *
 * Return value: %TRUE if at least a window has been opened
 **/
gboolean
ephy_session_autoresume (EphySession *session)

I dont think gtk-doc has a syntax document, though gtk is
a good example to look at.

- If it's a GObject add the get_type function and include
the .h in doc/reference/epiphany.types.

- Remove the .h from the IGNORE_HFILES section of
doc/reference/Makefile.am

- Clean current docs build using "make maintainer-clean-local"

- Build and commit both the modified .c and the sgml generated
in doc/reference/tmpl.