doc: Improve setup_clean_chain documentation

test/functional/README.md

@@ -63,10 +63,13 @@ don't have test cases for.
 - Avoid stop-starting the nodes multiple times during the test if possible. A
   stop-start takes several seconds, so doing it several times blows up the
   runtime of the test.
-- Set the `self.setup_clean_chain` variable in `set_test_params()` to control whether
-  or not to use the cached data directories. The cached data directories
-  contain a 200-block pre-mined blockchain and wallets for four nodes. Each node
-  has 25 mature blocks (25x50=1250 BTC) in its wallet.
+- Set the `self.setup_clean_chain` variable in `set_test_params()` to `True` to
+  initialize an empty blockchain and start from the Genesis block, rather than
+  load a premined blockchain from cache with the default value of `False`. The
+  cached data directories contain a 200-block pre-mined blockchain with the
+  spendable mining rewards being split between four nodes. Each node has 25
+  mature block subsidies (25x50=1250 BTC) in its wallet. Using them is much more
+  efficient than mining blocks in your test.
 - When calling RPCs with lots of arguments, consider using named keyword
   arguments instead of positional arguments to make the intent of the call
   clear to readers.