nit: document that `fout` must be open for reading too

diff --git a/src/src/receive.c b/src/src/receive.c
index 5125a4f47525fbb6af01d8b0add9cd255daea2df..ed2c57ba5e75fc507a9ad3187c9a2d9334e121f5 100644 (file)
--- a/src/src/receive.c
+++ b/src/src/receive.c
@@ -906,7 +906,8 @@ a cut-down version of the state-machine above; we don't need to do leading-dot
 detection and unstuffing.
 
 Arguments:
-  fout      a FILE to which to write the message; NULL if skipping
+  fout      a FILE to which to write the message; NULL if skipping;
+            must be open for both writing and reading.
 
 Returns:    One of the END_xxx values indicating why it stopped reading
 */