docs: core-api/printk-formats.rst: Clarify formatting {cpu,node}mask

Clarify how to pass the field width for bitmaps, and mention the helper
macros that are available to ease printing cpumask and nodemask.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201110144121.3278667-1-geert+renesas@glider.be


Signed-off-by: Jonathan Corbet <corbet@lwn.net>