fopen.3: Note the open(2) flags that correspond to the 'mode' argument

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/fopen.3

@@ -147,6 +147,24 @@ at end-of-file, as if preceded the call:
 
     fseek(stream, 0, SEEK_END);
 .fi
+.PP
+The file descriptor associated with the stream is opened as if by a call to
+.BR open (2)
+with the following flags:
+.RS
+.TS
+allbox;
+lb lb
+c l.
+fopen() mode	open() flags
+\fIr\fP	O_RDONLY
+\fIw\fP	O_WRONLY | O_CREAT | O_TRUNC
+\fIa\fP	O_WRONLY | O_CREAT | O_APPEND
+\fIr+\fP	O_RDWR
+\fIw+\fP	O_RDWR | O_CREAT | O_TRUNC
+\fIa+\fP	O_RDWR | O_CREAT | O_APPEND
+.TE
+.RE
 .\"
 .SS fdopen()
 The