README: update --enable-heterogeneous warning

Somehow the "do not use the heterogeneous!" warning ended up in the
wrong section (as evidenced by
#9697 (comment)).
Move it to the correct section, and also make it crystal clear that
the warning is about the heterogeneous functionality.

Signed-off-by: Jeff Squyres <[email protected]>

Author: jsquyres

README

@@ -1533,11 +1533,11 @@ MISCELLANEOUS FUNCTIONALITY
   with different endian representations).  Heterogeneous support is
   disabled by default because it imposes a minor performance penalty.
 
+  *** THE HETEROGENEOUS FUNCTIONALITY IS CURRENTLY BROKEN - DO NOT USE ***
+
  --enable-spc
   Enable software-based performance counters capability.
 
-  *** THIS FUNCTIONALITY IS CURRENTLY BROKEN - DO NOT USE ***
-
 --with-wrapper-cflags=<cflags>
 --with-wrapper-cxxflags=<cxxflags>
 --with-wrapper-fflags=<fflags>