Document that 021-glb-entry has only been done for CENTER_EBR_CIB tiles
for now (1200HC/2000HC devices).

fuzzers/machxo2/021-glb-entry/fuzzer.py

diff --git a/fuzzers/machxo2/021-glb-entry/fuzzer.py b/fuzzers/machxo2/021-glb-entry/fuzzer.py
index 36e8597..5ce8394 100644
--- a/fuzzers/machxo2/021-glb-entry/fuzzer.py
+++ b/fuzzers/machxo2/021-glb-entry/fuzzer.py
@@ -13,6 +13,9 @@
 
 
 def main():
+    # left_end and right_end are 1200HC-specific. However, the results
+    # also readily apply to 2000HC devices because they also have a
+    # CENTER_EBR_CIB tile (without qualifiers).
     def left_end(x):
         return 8 if x % 2 == 0 else 7