Dev Docs: Put RPC Tables In Separate CSS Class

Suggested by Saivann: RPC tables with Name/Type/Presence/Description
format are now in .ntpd CSS class so that we can format them specially.

We also change the style of all dev doc tables to replace bold table
headings (thead) with a double-line border-bottom.  This makes the
theads look less like section headings and prevents us from over-using
bold.

Minor: a Makefile warning message has been made more explicit (also
suggested by Saivann) and another Makefile rule was updated to catch
more broken tables.