Merge bitcoin/bitcoin#28157: test doc: tests acceptstalefeeestimates option is only supported on regtest chain

ee5a0369cc test: ensure acceptstalefeeestimates is supported only on regtest chain (ismaelsadeeq)
22d5d4b2b2 tx fees, policy: doc: update and delete unnecessary comment (ismaelsadeeq)

Pull request description:

  This PR Follow up comments from [#27622](https://github.com/bitcoin/bitcoin/pull/27622)

  It test that the new `regtest-only` option `acceptstalefeeestimates` is not supported on [main, signet and test chains](https://github.com/bitcoin/bitcoin/pull/27622/files#r1235218268), removes an unnecessary [comment](https://github.com/bitcoin/bitcoin/pull/27622/files#r1235204323), and update fee estimator  `MAXFILEAGE` [description comment](https://github.com/bitcoin/bitcoin/pull/27622/files#r1233887314).

ACKs for top commit:
  jonatack:
    ACK ee5a0369cc
  glozow:
    utACK ee5a0369cc

Tree-SHA512: 4755f25b08db62f37614ea768272b12580ee0d481fb7fa339379901a6132c66828777c6747d3fe67490ceace3a6ff248bf13bdf65720f6e5ba8642eb762acd3c

test/functional/feature_config_args.py

@@ -371,6 +371,14 @@ class ConfArgsTest(BitcoinTestFramework):
             f'is being used instead.') + r"[\s\S]*", env=env, match=ErrorMatch.FULL_REGEX)
         node.args = node_args
 
+    def test_acceptstalefeeestimates_arg_support(self):
+        self.log.info("Test -acceptstalefeeestimates option support")
+        conf_file = self.nodes[0].datadir_path / "bitcoin.conf"
+        for chain, chain_name in {("main", ""), ("test", "testnet3"), ("signet", "signet")}:
+            util.write_config(conf_file, n=0, chain=chain_name, extra_config='acceptstalefeeestimates=1\n')
+            self.nodes[0].assert_start_raises_init_error(expected_msg=f'Error: acceptstalefeeestimates is not supported on {chain} chain.')
+        util.write_config(conf_file, n=0, chain="regtest")  # Reset to regtest
+
     def run_test(self):
         self.test_log_buffer()
         self.test_args_log()
@@ -383,6 +391,7 @@ class ConfArgsTest(BitcoinTestFramework):
         self.test_invalid_command_line_options()
         self.test_ignored_conf()
         self.test_ignored_default_conf()
+        self.test_acceptstalefeeestimates_arg_support()
 
         # Remove the -datadir argument so it doesn't override the config file
         self.nodes[0].args = [arg for arg in self.nodes[0].args if not arg.startswith("-datadir")]