Merge bitcoin/bitcoin#29498: test: Update --tmpdir doc string to say directory must not exist

d4e36ae test: Update --tmpdir doc string to say directory must not exist (kevkevin)

Pull request description:

  The error message given if passing an existing dir to --tmpdir is confusing so this makes it clear that the directory must not already exist

  This change is motivated by this comment bitcoin/bitcoin#29335 (comment)

ACKs for top commit:
  maflcko:
    lgtm ACK d4e36ae
  davidgumberg:
    ACK bitcoin/bitcoin@d4e36ae

Tree-SHA512: fb31fd079767abbf94076615817943f35f5c9262fc97e65c631a18d33b3a343fe6a2d151613256e632d2b372ab2de0435f4712309b4a77ed3c663fd93a7dcdd1

Author: fanquake

test/functional/test_framework/test_framework.py

@@ -164,7 +164,7 @@ def parse_args(self):
                             help="Don't stop bitcoinds after the test execution")
         parser.add_argument("--cachedir", dest="cachedir", default=os.path.abspath(os.path.dirname(os.path.realpath(__file__)) + "/../../cache"),
                             help="Directory for caching pregenerated datadirs (default: %(default)s)")
-        parser.add_argument("--tmpdir", dest="tmpdir", help="Root directory for datadirs")
+        parser.add_argument("--tmpdir", dest="tmpdir", help="Root directory for datadirs (must not exist)")
         parser.add_argument("-l", "--loglevel", dest="loglevel", default="INFO",
                             help="log events at this level and higher to the console. Can be set to DEBUG, INFO, WARNING, ERROR or CRITICAL. Passing --loglevel DEBUG will output all logs to console. Note that logs at all levels are always written to the test_framework.log file in the temporary test directory.")
         parser.add_argument("--tracerpc", dest="trace_rpc", default=False, action="store_true",