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

Documentation/core-api/printk-formats.rst

@@ -531,7 +531,9 @@ For printing bitmap and its derivatives such as cpumask and nodemask,
 %*pb outputs the bitmap with field width as the number of bits and %*pbl
 output the bitmap as range list with field width as the number of bits.
 
-Passed by reference.
+The field width is passed by value, the bitmap is passed by reference.
+Helper macros cpumask_pr_args() and nodemask_pr_args() are available to ease
+printing cpumask and nodemask.
 
 Flags bitfields such as page flags, gfp_flags
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