bug#9054: 24.0.50; show source in other window

[Juri Linkov]

>> 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’
>
> I wasn't talking about C-h ...  I meant the context menu popping up via
> mouse-3 when 'context-menu-mode' is enabled.  I think whichever text a
> user clicks on in an Elisp buffer should pop up something informative.
> But with
>
> (context-menu-mode)
> (setq x 3)
> (setp x 3)
>
> right clicking on 'setq' and choosing "Describe symbol" from the menu
> displays a help buffer for 'setq'.  Doing the same for 'setp' gets me
>
> down-mouse-3 describe-symbol
>
> in the echo area.

But the context menu just calls describe-symbol, and describe-symbol
decides whether there is a documentation for the provided argument.
So isn't it the task of describe-symbol to signal an error?

Anyway, now if a symbol doesn't exist, then no menu item is added.
So no error message is needed.