DOC: Note assert_almost_equal impl. detail
Note the hard-coded switch between absolute and relative tolerance during checking.

Closes #21528.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21580?src=pr&el=h1) Report
> Merging [#21580](https://codecov.io/gh/pandas-dev/pandas/pull/21580?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f1ffc5fae06a7294dc831887b0d76177aec9b708?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/21580/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21580?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21580   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49549    49549           
=======================================
  Hits        45537    45537           
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21580?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21580/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.06% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21580?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/21580?src=pr&el=footer). Last update [f1ffc5f...0f4d201](https://codecov.io/gh/pandas-dev/pandas/pull/21580?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!