DOC: update docstring for series.nunique
Part of europandas sprint - expanded docstring for series.nunique @datapythonista 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=h1) Report
> Merging [#25116](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/800d38b90fe21d5aa87a60470611475fd6d9e604?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/25116/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25116   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52415    52415           
=======================================
  Hits        48418    48418           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <ø> (ø)` | :arrow_up: |
| #single | `42.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25116/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.75% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25116?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/25116?src=pr&el=footer). Last update [800d38b...1e496d0](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=h1) Report
> Merging [#25116](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/800d38b90fe21d5aa87a60470611475fd6d9e604?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/25116/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25116   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52415    52415           
=======================================
  Hits        48418    48418           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <ø> (ø)` | :arrow_up: |
| #single | `42.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25116/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.75% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25116?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/25116?src=pr&el=footer). Last update [800d38b...0a4bc50](https://codecov.io/gh/pandas-dev/pandas/pull/25116?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @cecilialiao  Thanks for accepting my first contribution! 