Replace (private) _checknull with (public) is_nan
`util._checknull(x)` is equivalent to `x is None or isnan(x)`.  On several occasions I have had to go remind myself what `_checknull` covers.  It is much clearer to write out `x is None or util.is_nan(x)`, which is what this PR does.

In at least one place where `_checknull` is currently used, the `is None` part of it can be skipped.

Also gets rid of usages of private functions, and trades a no-docstring func for a nice-docstring func.

Small assorted cleanups along the way.


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22146?src=pr&el=h1) Report
> Merging [#22146](https://codecov.io/gh/pandas-dev/pandas/pull/22146?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/def50e26f78c507a743fa11fee98230412ae50a8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22146/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22146?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22146   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         169      169           
  Lines       50694    50694           
=======================================
  Hits        46671    46671           
  Misses       4023     4023
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22146?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22146?src=pr&el=footer). Last update [def50e2...70e339e](https://codecov.io/gh/pandas-dev/pandas/pull/22146?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase and merge on green (just to be sure).