docs: netdev: refine 15-patch limit

The 15 patch limit is intended by the maintainers to cover
all outstanding patches on the mailing list on a per-tree basis.
Not just those in a single patchset. Document this practice accordingly.

Signed-off-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/process/maintainer-netdev.rst

···
 with better review coverage. Re-posting large series also increases the mailing
 list traffic.
 
+Limit patches outstanding on mailing list
+-----------------------------------------
+
+Avoid having more than 15 patches, across all series, outstanding for
+review on the mailing list for a single tree. In other words, a maximum of
+15 patches under review on net, and a maximum of 15 patches under review on
+net-next.
+
+This limit is intended to focus developer effort on testing patches before
+upstream review. Aiding the quality of upstream submissions, and easing the
+load on reviewers.
+
 .. _rcs:
 
 Local variable ordering ("reverse xmas tree", "RCS")