Be slightly more aggressive with erc-scrolltobottom-all

Be slightly more aggressive with erc-scrolltobottom-all

* lisp/erc/erc-goodies.el (erc--scrolltobottom-on-post-command):
Redo obsolete doc string.
(erc--scrolltobottom-at-prompt-minibuffer-active,
erc--scrolltobottom-on-win-conf-change): Rename former to latter to
better reflect actual role.  Remove conditional guard so it always
runs.
(erc--scrolltobottom-setup): Set `scroll-step' locally when a user
hasn't customized `scroll-conservatively'.  Update
`window-configuration-change-hook' member name.  (Bug#64855)