Document `&key'

[Paul Pogonyshev]

I stumbled upon the following function definition in a help buffer:

    (should-error FORM &rest KEYS &key TYPE EXCLUDE-SUBTYPES)

It would be helpful to have some help in the manual about `&key'. I
more or less understand what it means after searching for
`should-error' uses, but it would help if it was documented next to
`&rest' and `&optional'. I presume it is a CL extension?

Paul