video: fbdev: core: Fix kernel-doc warnings in fbmon + fb_notify

Fix kernel-doc warnings reported when using W=1

v2:
  - Improve subject (Lee)

v3:
  - Add RETURNS documentation (Thomas)

v4:
  - Use Return: not RETURNS (Randy)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Alexander A. Klimov" <grandmaster@al2klimov.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206205132.GA1869030@ravnborg.org