Improve description of Attributes

[JJ]

The problem

The definition of Attribute is kind of convoluted, with the source not reflecting actually what's in there. Most methods are defined in BOOTSTRAPATTR, and then baked into Attribute using the metamodel below.

The documentation also lists traits in two separate places, with some definitions repeated.

Suggestions

Revise thoroughly and improve where possible.

[lizmat]

Looks to me that the documentation of Attribute is pretty complete. Added documentation of methods I think are missing with ac9f8e9 .

Perhaps the traits issue requires a separate issue?

[lizmat]

@JJ unless you think some documentation is still missing, this can be closed afaiac

[JJ]

Please do so. Great job.