Timestamp variants are missing from documentation

[benjaminwinger]

There is just documentation for the TIMESTAMP type, but not also for

• TIMESTAMP_SEC
• TIMESTAMP_MS
• TIMESTAMP_NS
• TIMESTAMP_TZ

The TIMESTAMP docs also just mention millisecond precision, when internally it uses microsecond precision (probably more useful when documented in the context of the other variants). Similarly, we should probably document the limits of the different types TIMESTAMP being signed microseconds from 1970-01-01, or seconds from for TIMESTAMP_SEC, etc. (I don't recall what exactly we do with TIMESTAMP_TZ; I think it's stored the same and just formatted based on the local timezone).