[PATCH] Add observation that ports are not thread-safe

["Diogo F. S. Ramos" <dfsr@riseup.net>]

It's not obvious that ports are not thread-safe and trying to have
multiple threads writing to one returns errors that are not
recognizable as been caused by this lack of thread-safeness.

* doc/ref/guile.texi: non thread-safe observation
---
 doc/ref/api-io.texi | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/doc/ref/api-io.texi b/doc/ref/api-io.texi
index 02d92a2..e9f79ae 100644
--- a/doc/ref/api-io.texi
+++ b/doc/ref/api-io.texi
@@ -88,6 +88,10 @@ representation for output. There are three possible strategies: to
 raise an error, to replace the character with a hex escape, or to
 replace the character with a substitute character.
 
+Ports are not thread-safe.  A port has to be protected if you want to
+access it from multiple threads using mechanisms like mutexes.
+(@pxref{Mutexes and Condition Variables})
+
 @rnindex input-port?
 @deffn {Scheme Procedure} input-port? x
 @deffnx {C Function} scm_input_port_p (x)
-- 
1.9.1