Docfix.

* gnus.el (gnus-summary-line-format): Reference
`gnus-user-date-format-alist' for the &user-date; format, not
`gnus-summary-user-date-format-alist'.
* gnus-start.el (gnus-clean-old-newsrc): Always remove 'unexist' marks
if `gnus-newsrc-file-version' does not match `gnus-version'.  This
fixes a bug in Emacs trunk where the 'unexist' marks were always
removed at startup because "Gnus v5.13" was considered smaller than
"Ma Gnus v0.03".
gnus-sum.el (gnus-newsgroup-unexist): Spelling fix