Add table specifying what operators are valid for what searches

[bctcvai]

Some operations are only valid for some search types, either by design or omission. These should be documented.

One that is missing that aught to be added is $section ISIN [1,2,3] currently $section assumes a single id argument.

Additionally, now that Section objects have attributes it may be keen to add related_section to the grammar in addition to $related_states and $related_media. This would return at least Media that belong to matching sections (and potentially states/localziations if the association is possible).