execveat.2: Note the value of argv[0] passed to script interpreters

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/execveat.2

@@ -175,6 +175,29 @@ system call is also needed to allow
 to be implemented on systems that do not have the
 .I /proc
 filesystem mounted.
+
+When asked to execute a script file, the
+.IR argv[0]
+that is passed to the script interpreter is a string of the form
+.IR /dev/fd/N
+or
+.IR /dev/fd/N/P ,
+where
+.I N
+is the number of the file descriptor passed via the
+.IR dirfd
+argument.
+A string of the first form occurs when
+.BR AT_EMPTY_PATH
+is employed.
+A string of the second form occurs when the script is specified via both
+.IR dirfd
+and
+.IR pathname ;
+in this case,
+.IR P
+is the value given in
+.IR pathname .
 .SH BUGS
 The
 .B ENOENT