doc: tweak CSS for responsive table display

Noticed that <p> within a responsive table (as found in the kconfig docs
generated by the new genrest.py script in PR #20322) weren't displaying
using the same font size as table cells without <p> content. This
situation occurs when the help text in the Kconfig file is more than one
paragraph.

Also added a comment explaining why a previous CSS tweak was added.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>