Add note about bytecode compatibility wrt. LJ_GC64/LJ_FR2.

doc/extensions.html

@@ -207,6 +207,11 @@ for dot releases (x.y.0 → x.y.1), but may change with major or
 minor releases (2.0 → 2.1) or between any beta release. Foreign
 bytecode (e.g. from Lua 5.1) is incompatible and cannot be loaded.
 </p>
+<p>
+Note: <tt>LJ_GC64</tt> mode requires a different frame layout, which implies
+a different, incompatible bytecode format for ports that use this mode (e.g.
+ARM64). This may be rectified in the future.
+</p>
 
 <h3 id="table_new"><tt>table.new(narray, nhash)</tt> allocates a pre-sized table</h3>
 <p>