[AD] Document two more macros

[Eric Botcazou <ebotcazou@xxxxxxxxxx>]

The patch documents two undocumented macros used to insulate Allegro from the 
platform's peculiarities.

Applied to trunk and branch.

-- 
Eric Botcazou

Index: docs/src/allegro._tx
===================================================================
RCS file: /cvsroot/alleg/allegro/docs/src/allegro._tx,v
retrieving revision 1.149
diff -u -r1.149 allegro._tx
--- docs/src/allegro._tx	10 May 2003 20:53:00 -0000	1.149
+++ docs/src/allegro._tx	13 May 2003 16:55:46 -0000
@@ -9139,10 +9139,17 @@
    Use this in place of the regular "inline" function modifier keyword, and 
    your code will work correctly on any of the supported compilers.
 
+<b>RET_VOLATILE</b><br>
+   Use this to declare a function with a volatile return value.
+
 <b>ZERO_SIZE_ARRAY(type, name)</b><br>
    Use this to declare zero-sized arrays in terminal position inside 
    structures, like in the BITMAP structure. These arrays are effectively 
    equivalent to the flexible array members of ISO C99.
+
+<b>AL_CONST</b><br>
+   Use this in place of the regular "const" object modifier keyword, and 
+   your code will work correctly on any of the supported compilers.