bug#21601: 25.0.50; (elisp) `Common Keywords', :group

[Drew Adams]

> > Mention here, in the part about :group, and not just in node `Variable
> > Definitions', that if :group is not specified then the defcustom
> > inherits the group from the last :group of a preceding defcustom in the
> > same file.
> 
> I did now know that.  But is that something we want to encourage?

It's not something that I think should be encouraged.
But it is something that Stefan encourages.  And he
actively removes :group declarations that are not
strictly necessary.

As long as that is the practice, so that many defcustoms
and deffaces do not have :group (and that is increasingly
the case, for distributed Emacs code, at least), we should
document the behavior (IMHO).