gnus.el (gnus-registry-ignored-groups): Provide default in gnus.el, not gnus-registry.el.

gnus.el (gnus-registry-ignored-groups): Provide default in gnus.el, not gnus-registry.el.
gnus-registry.el (gnus-registry-ignored-groups): Remove defcustom.  Explain why in comments.
 (gnus-registry-action): Fix data-header reference to use the extra headers.  Explain in package commentary how to add To and Cc headers to the gnus-extra-headers.
 (gnus-registry-ignored-groups): Adjust defaults to match the parameter.
 (gnus-registry-ignore-group-p): Adjust to take either a group/topic parameter list or a string list in `gnus-registry-ignored-groups'.  Fix logic error.