Add hints for DB calls to docstrings

[fabiangeisler]

Each Shotgrid Database call slows down code execution, which means that some pysg functions are more efficient than others.
For example sg_project["code"].get() and sg_project.all_field_values() each do only one call to the Shotgrid Database, but the second call returns vastly more information and might be more desirable to use in your code.
It would be great for the user to see how many calls are made in the docstring of each function, so they can decide what to use and optimise easier.