kgdb,docs: Fix typo and minor style issues

Fix up a number of small typos, duplications and formatting issues.

Signed-off-by: Graham Whaley <graham.whaley@linux.intel.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
[jc: did s/kgdbdoc/kgdboc/]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>