Update proofs/cbmc/Makefile.common

Co-authored-by: Hanno Becker <beckphan@amazon.co.uk>
Signed-off-by: Roderick Chapman <rodchap@amazon.com>

Author: rod-chapman

proofs/cbmc/Makefile.common

@@ -542,7 +542,7 @@ COMMA :=,
 # CBMC team recommend that --no-array-field-sensitivity should never be used
 # with CBMC 6.9.0 and above when --slice-formula has been specified
 ifeq ($(findstring --no-array-field-sensitivity,$(CBMCFLAGS)),--no-array-field-sensitivity)
-$(error CBMC option --no-array-field-sensitivity must not be used with CBMC 6.9.0 and above)
+$(error CBMC option --no-array-field-sensitivity is believed to be detrimental to performance in CBMC v6.9.0 and has been removed from the CBMC Makefiles -- do you really want this?)
 endif
 
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