kgdb,docs: Update the kgdb docs to include kdb

Update the kgdb docs to reflect the new directory structure and API.

Merge in the kdb shell information.

[Randy Dunlap <rdunlap@xenotime.net>: grammatical corrections]

CC: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>