doc: Improve setup_clean_chain documentation

Author: fjahr

Diff for: 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.

Diff for: test/functional/example_test.py

@@ -76,6 +76,9 @@ def set_test_params(self):
         """Override test parameters for your individual test.
 
         This method must be overridden and num_nodes must be explicitly set."""
+        # By default every test loads a pre-mined chain of 200 blocks from cache.
+        # Set setup_clean_chain to True to skip this and start from the Genesis
+        # block.
         self.setup_clean_chain = True
         self.num_nodes = 3
         # Use self.extra_args to change command-line arguments for the nodes

Diff for: test/functional/test-shell.md

@@ -178,7 +178,7 @@ can be called after the TestShell is shut down.
 | `num_nodes` | `1` | Sets the number of initialized bitcoind processes. |
 | `perf` | False | Profiles running nodes with `perf` for the duration of the test if set to `True`. |
 | `rpc_timeout` | `60` | Sets the RPC server timeout for the underlying bitcoind processes. |
-| `setup_clean_chain` | `False` | Initializes an empty blockchain by default. A 199-block-long chain is initialized if set to `True`. |
+| `setup_clean_chain` | `False` | A 200-block-long chain is initialized from cache by default. Instead, `setup_clean_chain` initializes an empty blockchain if set to `True`. |
 | `randomseed` | Random Integer | `TestShell.options.randomseed` is a member of `TestShell` which can be accessed during a test to seed a random generator. User can override default with a constant value for reproducible test runs. |
 | `supports_cli` | `False` | Whether the bitcoin-cli utility is compiled and available for the test. |
 | `tmpdir` | `"/var/folders/.../"` | Sets directory for test logs. Will be deleted upon a successful test run unless `nocleanup` is set to `True` |