bug#24999: 24.5; bad doc for invocation-directory

[Francesco Potortì]

The elisp manual says:

 -- Variable: invocation-directory
     This variable holds the directory from which the Emacs executable
     was invoked, or perhaps `nil' if that directory cannot be
     determined.

However, the doc string says:

invocation-directory is a variable defined in `C source code'.
Its value is "/usr/bin/"

Documentation:
The directory in which the Emacs executable was found, to run it.
The value is nil if that directory's name is not known.


The second description is the correct one, and the elisp manual is
wrong.

The only way I found for obtaining the directory from which
Emacs was invoked is to get the default-directory of the *scratch*
buffer.