doc: cyclic: remove reference to WATCHDOG_RESET

doc: cyclic: remove reference to WATCHDOG_RESET

WATCHDOG_RESET is no more. Replace the reference by schedule().

While here, rearrange the sentence a bit so that "cyclic_run()"
becomes the object and "the main function responsible for calling all
registered cyclic functions" a parenthetical rather than the other way
around, which at least to me makes it more readable.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>