bug#42704: Documentation of call-process

[Gregory Heytings]

> > The documentation of 'call-process' doe not make reference to the 
> > variable 'exec-path'.  It should.
>
> The ELisp manual says that, see the node "Subprocess Creation".  This 
> isn't specific to call-process, that variable is used by all the 
> functions that create subprocesses.

Indeed.  I still believe that adding the sentence "If PROGRAM is not an 
absolute file name, the variable exec-path contains a list of directories 
to search." in the docstring of call-process (and a similar sentence in 
the docstrings of make-process and call-process-region) would be useful.

> > Using '(setenv "PATH" "...")' does not have the expected effect for 
> > 'call-process'.  The environment variable "PATH" is correctly updated, 
> > the variable 'process-environment' is also updated, but what 
> > 'call-process' actually uses to locate the executable is the variable 
> > 'exec-path'.
>
> The changes to the environment variables only affect the environment 
> passed to subprocesses, they don't affect the environment of the Emacs 
> process itself.

Indeed.  Creating a subprocess is in between the two (Emacs and the 
subprocess), and I at least would have expected that setting "PATH" would 
have changed the path in which the program executable is searched.

Gregory