Improve docs of maxschedchunk (#893)

docs/distribution.rst

@@ -50,7 +50,8 @@ The test distribution algorithm is configured with the ``--dist`` command-line o
 .. _distribution modes:
 
 * ``--dist load`` **(default)**: Sends pending tests to any worker that is
-  available, without any guaranteed order.
+  available, without any guaranteed order. Scheduling can be fine-tuned with
+  the `--maxschedchunk` option, see output of `pytest --help`.
 
 * ``--dist loadscope``: Tests are grouped by **module** for *test functions*
   and by **class** for *test methods*. Groups are distributed to available

src/xdist/plugin.py

@@ -173,6 +173,8 @@ def pytest_addoption(parser):
             "one - might be useful for a small number of slow tests. "
             "Larger numbers will allow the scheduler to submit consecutive "
             "chunks of tests to workers - allows reusing fixtures. "
+            "Due to implementation reasons, at least 2 tests are scheduled per "
+            "worker at the start. Only later tests can be scheduled one by one. "
             "Unlimited if not set."
         ),
     )