(Ports and File Descriptors): Tweaks to fcntl.

doc/ref/posix.texi

@@ -472,10 +472,11 @@ If @var{size} is omitted, a default size will be used.
 @end defvar
 @end deffn
 
-@deffn {Scheme Procedure} fcntl object cmd [value]
+@deffn {Scheme Procedure} fcntl port/fd cmd [value]
 @deffnx {C Function} scm_fcntl (object, cmd, value)
-Apply @var{cmd} on @var{object}, either a port or file descriptor.
-The @var{value} is an integer argument, for the @code{SET} commands.
+Apply @var{cmd} on @var{port/fd}, either a port or file descriptor.
+The @var{value} argument is used by the @code{SET} commands described
+below, it's an integer value.
 
 Values for @var{cmd} are:
 
@@ -497,6 +498,13 @@ flag,
 @example
 (fcntl port F_SETFD FD_CLOEXEC)
 @end example
+
+Or better, set it but leave any other possible future flags unchanged,
+
+@example
+(fcntl port F_SETFD (logior FD_CLOEXEC
+                            (fcntl port F_GETFD)))
+@end example
 @end defvar
 @end defvar
 
@@ -509,8 +517,8 @@ A common use is to set @code{O_NONBLOCK} on a network socket.  The
 following sets that flag, and leaves other flags unchanged.
 
 @example
-(fcntl sock F_SETFL
-       (logior (fcntl sock F_GETFL) O_NONBLOCK))
+(fcntl sock F_SETFL (logior O_NONBLOCK
+                            (fcntl sock F_GETFL)))
 @end example
 @end defvar