doc/developer: Add terse documentation of assumed C implementation defined behavior

Suggested-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/developer.texi

@@ -131,6 +131,11 @@ designated struct initializers (@samp{struct s x = @{ .i = 17 @};});
 
 @item
 compound literals (@samp{x = (struct s) @{ 17, 23 @};}).
+
+@item
+Implementation defined behavior for signed integers is assumed to match the
+expected behavior for two's complement. Non representable values in integer
+casts are binary truncated. Shift right of signed values uses sign extension.
 @end itemize
 
 These features are supported by all compilers we care about, so we will not