fw_cfg: move internal function call docs to header file

fw_cfg: move internal function call docs to header file

Move documentation for fw_cfg functions internal to qemufrom
docs/specs/fw_cfg.txt to the fw_cfg.h header file, next to
their prototype declarations, formatted as doc-comments.

NOTE: Documentation for fw_cfg_add_callback() is completely
dropped by this patch, as that function has been eliminated
by commit 023e3148.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc Marí <markmb@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1446733972-1602-2-git-send-email-somlo@cmu.edu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>