Adds documentation for all of the date time functions using the new
cli-like format extracted from the csv spec. In the process of doing
this I noticed that the `WEEK` function isn't exposed as a function.
This exposes it for consistency.
Relates to elastic/x-pack-elasticsearch#2898
Original commit: elastic/x-pack-elasticsearch@0459b24cb9
I went to write some docs for datetime functions that look like:
```
SELECT YEAR(CAST('2018-01-19T10:23:27Z' AS TIMESTAMP)) as year;
year
2018
```
because I figured they'd be pretty easy to read because they didn't
require any knowledge of a data set. But it turns out that constant
folding doesn't work properly for date time functions because they don't
actually apply the extraction.
Original commit: elastic/x-pack-elasticsearch@aa9c66b2c7
Nik Everett