Write documentation & CEP for all the Jinja functions and variables

[wolfv]

It would be good to have docs for all Jinja variables and also a CEP.

There are some features that have little documentation in conda-build, like pin_compatible and where it takes the pins from.

If I understand the code correctly, pin_compatible can only work properly in the run requirements (or below), for example. In conda-build m.is_cross is true whenever there is a host-section -> some more investigation and related cb-code: #368

We should fully specify all these functions and make sure that the implementation matches.

[swarnimarun]

Should be partially fixed after #483.