Define DPC at least one place in documentation

[tisaac]

Just about all implemented elements that have a commonly used acronym have that acronym expanded somewhere in the documentation, but I could not find such an expansion for DPC anywhere.

I think that a user who has not seen femtable before would not be able to tell you what DPC stands for. And it seems like a waste of time to direct them to defelement.com for such a simple to understand element.

The most visible part of the interface where this pops up is enum dpc_variant: I would recommend expanding on what is meant by DPC there.