DOC/DEPR: clarify argmin/argmax deprecation message
Attempt to better phrase the message, xref https://github.com/pandas-dev/pandas/issues/16830#issuecomment-412211029
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22310?src=pr&el=h1) Report
> Merging [#22310](https://codecov.io/gh/pandas-dev/pandas/pull/22310?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/188a4fc5535ea9022d086d379cbcaf562c1123f6?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/22310/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/22310?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22310   +/-   ##
=======================================
  Coverage   92.08%   92.08%           
=======================================
  Files         169      169           
  Lines       50691    50691           
=======================================
  Hits        46681    46681           
  Misses       4010     4010
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.49% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22310?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/22310/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.11% <ø> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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