Resolve oddities in `Generic.Ideal` documentation and implementation

[fingolfin]

• the source code says that it implements ideals over Euclidean domains
• but the manual claims that also "univariate or multivariate polynomial ring over the integers" are supported
• though maybe "over the integers" should be "over fields"? Then the next sentence would make more sense: "Univariate and multivariate polynomial rings over other domains (other than fields) are not supported at this time."
• there is no Base.in method, arguably something rather central
• there is no Base.isubset(I,J) method to test containment
• there is a Base.contains(I,J) method which really should be a Base.issubset method
• a bunch of the code seems to expect that all ideals are principle ideals - which is fine if one requires euclidean domains, but the docs claim (and some tests check) support for multiviariate polynomial rings...

[thofma]

Just for clarification. The implementation is for ideals in polynomials rings $R[x_1,\dotsc,x_n]$, where $R$ is a Euclidean domain, although there are some details in the implementation, which makes it only work $R = \mathbf{Z}$ or $R$ a field if I remember correctly.