Revise section 4, include new subsection on leap seconds, define utc and tai calendars and leap_seconds in units_metadata

[JonathanGregory]

See issue #542 for discussion of these changes.

Release checklist

• [NA] Authors updated in cf-conventions.adoc? Add in two places: on line 3 and under .Additional Authors in About the authors.
• [NA] Next version in cf-conventions.adoc up to date? Versioning inspired by SemVer.
• [Y] history.adoc up to date?
• [Y] Conformance document up to date?

[ChrisBarker-NOAA]

A few suggestions -- none of the show-stoppers -- do with them what you will :-)

I commente din one place, but everywhere, I"d prefer:

"datetime" or "date-time" over "date/time"

But if this has already been thoroughly hashed out -- no big deal.

[ChrisBarker-NOAA]

looks great, thanks!

[chris-little]

@davidhassell @JonathanGregory You might want to consider removing the word midnight, or replace it with midnight at 0 degrees longitude. It is a bit UK-centric. The ISO 8601 standard removed that word from its content some years ago.
Otherwise, a good clear exposition of the topics.

[ChrisBarker-NOAA]

This is outstanding! Thanks for all the hard work -- quite a slog, but the CF community will greatly benefit.