PROPOSAL: Custom help text for FilterSet

[snowman2]

What are your thoughts on adding optional help_text to the FilterSet.

Here is an example of how we use it:

class AttributeFilter(FilterSet):
    class Meta(object):
        model = Attribute
        fields = {
            "id": ["exact", "in"],
            "name": ["exact", "iexact", "icontains", "contains"],
        }
        help_text = {
            "id": {
                "exact": "id/primary key for an Attribute entry.",
                "in": "Comma separated list of ids/primary keys for a set of Attribute entries.",
            },
            "name": {
                "exact": "Attribute name field.",
                "iexact": "Case insensitive search for attribute name field.",
                "icontains": "Case insensitive `like` search for Attribute names.",
                "contains": "Case sensitive `like` search for Attribute names.",
            },
        }

This is useful when auto-generating API documentation.

cc: @alfredoahds

Note: I should add that we would be willing to submit a PR if you would like to see this added.