Fsubstitute_in_file_name doc string clarification

* src/fileio.c (Fsubstitute_in_file_name): Mention that undefined
variables aren't replaced (as opposed to what happens in a shell
substitution) (bug#40949).

src/fileio.c

@@ -1830,6 +1830,8 @@ the value of that variable.  The variable name should be terminated
 with a character not a letter, digit or underscore; otherwise, enclose
 the entire variable name in braces.
 
+If FOO is not defined in the environment, it is not substituted.
+
 If `/~' appears, all of FILENAME through that `/' is discarded.
 If `//' appears, everything up to and including the first of
 those `/' is discarded.  */)