bug#21139: 25.0.50; shell-mode doesn't respect comint-scroll-to-bottom-on-output/input

[Artur Malabarba]

1. Set `comint-scroll-to-bottom-on-output` and
`comint-scroll-to-bottom-on-input` to nil via the customize interface
(or via `setq-default`)
2. Run `M-x shell`
3. Run a couple of shell commands so you have enough output to fill
the screen (`ls -al ~/` should do it).
4. Hit `C-l` twice so point is at the top of the screen.
5. Type `date` on the prompt and hit `RET`.
6. The window scrolls until point is at the bottom.


It would seem that, by setting those variables to nil, the window
should not scroll. If this is not intended to be the case, then these
variables' docstrings are unclear to me.