Documentation/process/howto: Update for 4.x -> 5.x versioning

As linux-5.0 is coming up soon, the howto.rst document can be
updated for the new kernel version. Instead of changing all 4.x
references to 5.x, this time we git rid of all explicit version
numbers and rework some kernel trees' name to keep the docs
current and real.

Signed-off-by: Zenghui Yu <zenghuiyu96@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>