After more experimentation I found that this worked quite well:
# @!method size
# @see Array#size
# @!method <<
# @see Array#<<
# @!method blah
# @see Array#blah
def_delegators :@xs, :size, :<<, :blah # …
There is quite possibly a way to do this in one or two lines, but compared to the work writing an extension I find this very acceptable.
Update:
I've just found this will link to the delegated methods' documentation better:
# @!method size
# @return (see Array#size)
This would take the already documented return value from the Array#size method. I expect the other tags will do this too. It's still quite verbose, but acceptable.