bug#9976: Not on a date!

[Juri Linkov]

Instead of typing `C-h f RET' to get the documentation of the function
under cursor, I mistakenly typed `C-h d RET' and got in the echo area
the message "Not on a date!".  With relevant settings from .emacs this
can be reproduced with the following test case:

1. emacs -Q
2. Eval: (require 'calendar)
3. Eval: (setq apropos-do-all t)
4. `C-h d RET' or `C-u C-h d RET'
5. See the message "Not on a date!"

That's because `apropos-documentation-check-doc-file' calls
`substitute-command-keys' that calls `cal-menu-set-date-title'
that displays this message.

I don't know if this counts as a bug, but seeing this message is confusing.