bug#9054: 24.0.50; show source in other window

[Juri Linkov]

>>> Consider with
>>>
>>> (setq x 3)
>>>
>>> clicking on "x" or "3".
>>
>> There is already feedback when there is no Manual entry:
>>
>>    (error "Not documented as a symbol:")
>
> I only see
>
> down-mouse-3 describe-symbol
>
> in the echo area.

Every help command has own unique feedback:

  C-h f xyz C-M-c => error: Symbol’s function definition is void: xyz
  C-h v xyz C-M-c => Help buffer: xyz is void as a variable.
  C-h o xyz C-M-c => no output
  C-h w xyz C-M-c => message: xyz is not on any key
  C-h x xyz C-M-c => error: Symbol is not a command: xyz
  C-h a xyz C-M-c => message: No apropos matches for ‘xyz’