doc: what a patch series is

It seems that we need to clarify that a patch series is a series of related
patches rather than "here are some of my patches as multiple (numbered)
emails."

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>