Consider defining the ement-room-mode-hook docstring explicitly

I think this is an Emacs bug which could be resolved with a call to `format-mode-line` , but the auto-generated docstring for major mode hooks doesn't cope with the (entirely valid) use of mode line constructs in `mode-name`, so we end up with a \*Help\* buffer like this:

```plain
ement-room-mode-hook is a variable defined in ‘ement-room.el’.

Its value is (visual-line-mode)

Hook run after entering ‘(Ement-Room (:eval (unless (map-elt ement-syncs ement-session) (propertize :Not-syncing ’face ’font-lock-warning-face ’help-echo Automatic syncing was interrupted; press "g" to resume)))) mode.
No problems result if this variable is not bound.
‘add-hook’ automatically binds it. (This is true for all hook variables.)

This variable may be risky if used as a file-local variable.

[back]
```

I see that `emacs-lisp-mode` circumvents this with a `defcustom` for the hook, to declare a plainer-text docstring (although that might well have pre-dated the use of mode line constructs in its `mode-name`; I didn't check.)

It might be a sensible idea to do something similar in this case.