DOC: update the pandas.Index.notna docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

```
################################################################################
######################## Docstring (pandas.Index.notna) ########################
################################################################################

Detect existing (non-missing) values.

Return a boolean same-sized object indicating if the values are not NA.
Non-missing values get mapped to True. Characters such as empty
strings `''` or :attr:`numpy.inf` are not considered NA values
(unless you set :attr:`pandas.options.mode.use_inf_as_na` `= True`).
NA values, such as None or :attr:`numpy.NaN`, get mapped to False
values.

.. versionadded:: 0.20.0

Returns
-------
bool of type numpy.ndarray
    Boolean array to indicate which entries are not NA.

See also
--------
notnull : alias of notna
isna: inverse of notna
pandas.notna : top-level notna

Examples
--------
Show which entries in a pandas.Index are not NA. The result is an
array.

>>> idx = pd.Index([5.2, 6.0, np.NaN])
>>> idx
Float64Index([5.2, 6.0, nan], dtype='float64')
>>> idx.notna()
array([ True,  True, False])

Empty strings are not considered NA values. None is considered a NA
value.

>>> idx = pd.Index(['black', '', 'red', None])
>>> idx
Index(['black', '', 'red', None], dtype='object')
>>> idx.notna()
array([ True,  True,  True, False])

################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.Index.notna" correct. :)
```

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```diff
@@            Coverage Diff            @@
##             master   #20180   +/-   ##
=========================================
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=========================================
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  Branches          ?        0           
=========================================
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```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.08% <ø> (?)` | |
| #single | `41.84% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20180?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20180/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <ø> (ø)` | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20180?src=pr&el=footer). Last update [d7bcb22...e6aad42](https://codecov.io/gh/pandas-dev/pandas/pull/20180?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @donK23 made a minor edit, thanks a lot!