Add documentation for `statistics()` and `aclose()` on memory channels

[A5rocks]

I'm pretty sure the reason Sphinx isn't showing them is because they have no docstrings. Easy fix.

Mentioned on Gitter.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.58%. Comparing base (498325d) to head (da4882c). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3101 +/- ## ======================================= Coverage 99.58% 99.58% ======================================= Files 121 121 Lines 18002 18002 Branches 3248 3248 ======================================= Hits 17927 17927 Misses 52 52 Partials 23 23 ``` | [Files with missing lines](https://app.codecov.io/gh/python-trio/trio/pull/3101?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_\_init\_\_.py](https://app.codecov.io/gh/python-trio/trio/pull/3101?src=pr&el=tree&filepath=src%2Ftrio%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX19pbml0X18ucHk=) | `100.00% <ø> (ø)` | | | [src/trio/\_channel.py](https://app.codecov.io/gh/python-trio/trio/pull/3101?src=pr&el=tree&filepath=src%2Ftrio%2F_channel.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX2NoYW5uZWwucHk=) | `100.00% <100.00%> (ø)` | |

[A5rocks]

pre-commit.ci autofix