* gnus-registry.el: added brief explanation of basics

* gnus-registry.el: added brief explanation of basics
(gnus-registry-track-extra): new variable for tracking of message
subjects
(gnus-registry-entry-caching): caching parameter, used for extra
data
(gnus-registry-minimum-subject-length): minimum subject length
before it's considered when tracing subjects
(gnus-registry-save): accomodate extra data entry caching
(gnus-registry-action): change function name, add the subject and
pass it to gnus-registry-add-group
(gnus-registry-spool-action): change function name, add the
subject and pass it to gnus-registry-add-group
(gnus-registry-split-fancy-with-parent): add subject tracking
(gnus-registry-register-message-ids): pass subject to
gnus-registry-add-group
(gnus-registry-simplify-subject)
(gnus-registry-fetch-simplified-message-subject-fast): new
functions
(gnus-registry-fetch-extra, gnus-registry-fetch-extra-entry): add
extra data entry caching
(gnus-registry-add-group): handle the extra subject parameter
(gnus-registry-install-hooks, gnus-registry-unload-hook): fix the
gnus-register-* function names

* nnmail.el (nnmail-cache-insert): add subject parameter, pass it
on to the nnmail-spool-hook

* nnbabyl.el (nnbabyl-request-accept-article): added subject to
nnmail-cache-insert call

* nndiary.el (nndiary-request-accept-article): added subject to
nnmail-cache-insert call

* nnfolder.el (nnfolder-request-accept-article): added subject to
nnmail-cache-insert call

* nnimap.el (nnimap-split-articles): added subject to
nnmail-cache-insert call
(nnimap-request-accept-article): added subject to
nnmail-cache-insert call

* nnmbox.el (nnmbox-request-accept-article): added subject to
nnmail-cache-insert call

* nnmh.el (nnmh-request-accept-article): added subject to
nnmail-cache-insert call

* nnml.el (nnml-request-accept-article): added subject to
nnmail-cache-insert call