Documentation/kokr/howto: Wordsmith

The sentence regarding version numbers of '-stable' kernels is quite
ambiguous.  This commit makes the sentence more clear and fix
inconsistent uses of the terms for 'version'.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Link: https://lore.kernel.org/r/20200829082343.2979-3-sj38.park@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>