Fix small problems around eglot's help buffer

Fix small problems around eglot's help buffer

Specifically:

- correctly format the message shown to the user about doc being truncated
- don't show message if the buffer is showing in some frame's window
- correctly name the help buffer switched to with `C-h .'.

This is still not ideal:

- When the `C-h .' suggestion is shown to the user, typing that keybinding
  shouldn't result in a new LSP request to fetch probably the same info;
- All this functionality belongs in eldoc.el.

* eglot.el (eglot-help-at-point): Fix buffer name.
(eglot--update-doc): Provide more help.