doc(rs): some docs for the ClickHouse batch module

[onewland]

I wanted to check my own understanding here, and if people think it's useful then we can go ahead and merge.

[untitaker]

all looks good but references to this constant need fixing

[codecov]

Test Failures Detected: Due to failing tests, we cannot provide coverage reports at this time.

❌ Failed Test Results:

Completed 95 tests with 1 failed, 94 passed and 0 skipped.

View the full list of failed tests

Test Description	Failure message
Testsuite: pytest Test name: tests.test_cleanup.TestCleanup::test_midnight_error_case[transactions] Envs: - default	Traceback (most recent call last): File ".../snuba/tests/test_cleanup.py", line 218, in test_midnight_error_case assert [(p.date, p.retention_days) for p in parts] == [ AssertionError: assert [] == [(datetime.datetime(2021, 10, 25, 0, 0), 90)] Right contains one more item: (datetime.datetime(2021, 10, 25, 0, 0), 90) Full diff: - [(datetime.datetime(2021, 10, 25, 0, 0), 90)] + []