Documentation: bring process docs up to date

The guide to the kernel dev process documentation, for example, contains
references to older kernels and their timelines. In addition, one of the
"long term support kernels" listed have since reached EOL, and a new one
has been named. This patch brings information/tables up to date.

Additionally, some very trivial grammatical errors, unclear sentences,
and potentially unsavory diction have been edited.

Signed-off-by: Tony Fischetti <tony.fischetti@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>