Added warning about behavior of open mode 'a' (or 'a+').
diff --git a/Doc/libfuncs.tex b/Doc/libfuncs.tex
index 34d2928..088f969 100644
--- a/Doc/libfuncs.tex
+++ b/Doc/libfuncs.tex
@@ -265,7 +265,10 @@
\code{fopen()}: \var{filename} is the file name to be opened,
\var{mode} indicates how the file is to be opened: \code{'r'} for
reading, \code{'w'} for writing (truncating an existing file), and
- \code{'a'} opens it for appending. Modes \code{'r+'}, \code{'w+'} and
+ \code{'a'} opens it for appending (which on {\em some} {\UNIX}
+ systems means that {\em all} writes append to the end of the file,
+ regardless of the current seek position).
+ Modes \code{'r+'}, \code{'w+'} and
\code{'a+'} open the file for updating, provided the underlying
\code{stdio} library understands this. On systems that differentiate
between binary and text files, \code{'b'} appended to the mode opens