(mime-entity-header-buffer): Mark it as obsolete.

(mime-entity-header-buffer): Mark it as obsolete.
(mime-goto-header-start-point): Likewise.
(mime-entity-header-start-point): Likewise.
(mime-entity-header-end-point): Likewise.
(mime-entity-body-start): Use `defalias'; don't recommend to use
`mime-entity-body-start-point' instead.
(mime-entity-body-end): Use `defalias'; don't recommend to use
`mime-entity-body-end-point' instead.
(mime-entity-body-buffer): Mark it as obsolete.
(mime-goto-body-start-point): Likewise.
(mime-goto-body-end-point): Likewise.
(mime-entity-body-start-point): Likewise.
(mime-entity-body-end-point): Likewise.
(mime-entity-buffer): Don't recommend to use
`mime-entity-header-buffer' or `mime-entity-body-buffer' instead.
(mime-entity-point-min): Don't recommend to use
`mime-entity-header-start-point' instead.
(mime-entity-point-max): Don't recommend to use
`mime-entity-body-end-point' instead.