DOC: Fix documentation for read_csv's mangle_dupe_cols (GH19203)
- [ x] closes #19203
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=h1) Report
> Merging [#19208](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8347ff8c9d8876fa46f73dbc48919a672caef1d2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19208/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19208      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         147      147              
  Lines       48797    48812      +15     
==========================================
+ Hits        44664    44678      +14     
- Misses       4133     4134       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.9% <ø> (ø)` | :arrow_up: |
| #single | `41.7% <ø> (+0.1%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/19208/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.48% <ø> (ø)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/19208/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `87.98% <0%> (-0.68%)` | :arrow_down: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19208/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.5% <0%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19208/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `96.12% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/19208/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.38% <0%> (+0.08%)` | :arrow_up: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/19208/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `95.32% <0%> (+0.77%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=footer). Last update [8347ff8...918e94a](https://codecov.io/gh/pandas-dev/pandas/pull/19208?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Change looks good! Can you check if a similar edit in io.rst is needed? @Bernhard10 Thanks!