pdfork.2: add EFAULT as possible error, explain some consequences of it

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D55306

lib/libsys/pdfork.2

@@ -205,6 +205,24 @@ may return the same error numbers as
 .Xr fork 2 ) ,
 with the following additions:
 .Bl -tag -width Er
+.It Bq Er EFAULT
+The copyout of the resulting file descriptor value to the memory pointed
+to by
+.Fa fdp
+failed.
+.Pp
+Note that the child process was already created when this condition
+is detected,
+and the child continues execution, same as the parent.
+If this error must be handled, it is advisable to memoize the
+.Fn getpid
+result before the call to
+.Fn pdfork
+or
+.Fn pdrfork ,
+and compare it to the value returned by
+.Fn getpid
+after, to see if code is executing in parent or child.
 .It Bq Er EINVAL
 The signal number given to
 .Fn pdkill