Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-registry.el: Fix and extend header docs.  Move the nnregistry feature check.
nnimap.el (nnimap-update-info): Ignore groups that have no UIDNEXT values.
nnimap.el: Implement nnimap-request-newgroups.
gnus-group.el (gnus-group-completing-read): Return the symbol name, not the value from the collection.
nnimap.el: Don't error out on empty non-UIDNEXT groups.
nnmail.el (nnmail-article-group): Allow using the fancy split method directly.
gnus.el (gnus-continuum-version): Make inactive, since it doesn't really message anything to the user.
gnus-msg.el (gnus-summary-resend-message): When resending, don't externalize attachments.
gnus.el (gnus-large-newsgroup): Mention gnus-large-ephemeral-newsgroup.
gnus-sum.el (gnus-summary-scroll-up): Add more documentation.
message.el (message-shorten-references): Comment on the number "21".
mm-encode.el (mm-content-transfer-encoding-defaults): Try to make the documentation clearer.
ChangeLog: jidanni -> Dan Jacobson.
nnmbox.el (nnmbox-read-mbox): Mark buffer for deletion on Gnus exit.
gnus-sum.el, gnus-win.el: Change the `h' command to only show the article buffer.
gnus-art.el (gnus-summary-save-in-file, gnus-summary-save-in-rmail): Use with-current-buffer instead of gnus-eval-in-buffer-window to avoid popping up frames.
gnus-sum.el (gnus-summary-clear-local-variables): Removed.
gnus-async.el (gnus-async-delete-prefetched-entry): Remove from hash table, too.
gnus-async.el: Try to keep the async hash table size reasonable.
nndoc.el (nndoc-type-alist): Do babyl before mime-parts.
gnus-start.el (gnus-check-bogus-newsgroups): Say how many groups we're being queried about.