[Rivet] Source code documentation for 2.1.1

[Frank Siegert]

Hi Riveters,

It seems, our source code documentation for Rivet 2.1.1
(https://rivet.hepforge.org/code/dev/) is sourced from the *default*
branch, correct? For example, the DressedLeptons class already has a
constructor based on the new cuts system there.

Is this on purpose, or should we keep this generated from the
*release* branch, such that it can be referred to by analysis authors?

Cheers,
Frank