Added warning about behavior of open mode 'a' (or 'a+').

Doc/lib/libfuncs.tex

@@ -265,7 +265,10 @@ any kind of sequence; the result is always a list.
   \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

Doc/libfuncs.tex

@@ -265,7 +265,10 @@ any kind of sequence; the result is always a list.
   \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