Add some kind of informative summary table of extended attributes

[domenic]

I'm thinking the three columns are: name, applies to, notes.

An example entry would be: [Constructor], interfaces, multiple can be specified; conflicts with [NoInterfaceObject]; cannot be specified on callback interfaces.

This should be done after #286 as that would change the table significantly.

This would help massively for answering questions like the ones that came up during #286 around "which extended attributes apply to operations" and such.

Maybe two tables, one for ones we encourage, and one for ones we discourage.

[tobie]

We might also somehow group those in sections to make the ToC look less daunting (maybe "Types", "Interfaces only", "Others").