documentation incomplete: Content.cachable

[Gerrit Hohl]

The cachable property of the Content class is not explained, only mentioned a few times in various parts of the documentation.

http://www.web-polygraph.org/docs/reference/pgl/types.html#type:docs/reference/pgl/types/Content

I guess it is how many of the created objects should be cachable. But I don't how that works in detail and in collaboration with other parameters.