Add browse functions methods and their variables to documentation

[lubojr]

https://docs.eoxserver.org/en/latest/users/coverages.html#browse-type Outlines what is a browse type, but with recent improvements in offered browse functions and their on-the-fly rendering usage via variables overall, it is not documented well enough how to use them.

Documentation of these methods is partially included in VS Operator's guide, but ideally a thorough list should be listed in EOxServer docs and only linked from VS.