Document that the open pragma does not affect

the three-headed hydra, ummm, open.

p4raw-id: //depot/perl@19245

diff --git a/lib/open.pm b/lib/open.pm
index c294093..39e194f 100644 (file)
--- a/lib/open.pm
+++ b/lib/open.pm
@@ -166,9 +166,12 @@ Perl is configured to use PerlIO as its IO system (which is now the
 default).
 
 The C<open> pragma serves as one of the interfaces to declare default
-"layers" (also known as "disciplines") for all I/O. Any open(),
-readpipe() (aka qx//) and similar operators found within the lexical
-scope of this pragma will use the declared defaults.
+"layers" (also known as "disciplines") for all I/O. Any two-argument
+open(), readpipe() (aka qx//) and similar operators found within the
+lexical scope of this pragma will use the declared defaults.
+Three-argument opens are not affected by this pragma since there you
+(can) explicitly specify the layers and are supposed to know what you
+are doing.
 
 With the C<IN> subpragma you can declare the default layers
 of input streams, and with the C<OUT> subpragma you can declare