Better documentation about ignore_fields, include_fields

[sgillies]

We should be more clear that "desired fields" capabilities of layers are rare (see #1279) and that a DriverError must be expected in many cases.

Specifically, extra documentation is needed at

Fiona/fiona/__init__.py

Lines 183 to 188 in 75d2ab1

	ignore_fields : list
	List of field names to ignore on load.
	ignore_geometry : bool
	Ignore the geometry on load.
	include_fields : list
	List of a subset of field names to include on load.

.