[ADQL-2.x] Function cot(x)

[Grégory Mantelet]

Hi DAL,

I just want to mention an undocumented function in ADQL-2.0 as well as 
ADQL-2.1.

The trigonometric function `cot(x double) -> double` is not listed in 
the table
of section 2.3, although it is part of the BNF of v2.0 and v2.1.

I think it is just an oversight which could be easily fixed in the ADQL-2.1
document while it is still in the PR stage.

Cheers,
Grégory Mantelet