DOC: update the pandas.Index.notna docstring

[datadonK23]

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

• PR title is "DOC: update the docstring"
• The validation script passes: scripts/validate_docstrings.py <your-function-or-method>
• The PEP8 style check passes: git diff upstream/master -u -- "*.py" | flake8 --diff
• The html version looks good: python doc/make.py --single <your-function-or-method>
• 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. :)

[kynan]

``True``

[datadonK23]

Thx for the trick with the double quotes. I'm using it now.

[kynan]

``''``

[kynan]

``False``

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d7bcb22). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #20180   +/-   ##
=========================================
  Coverage          ?    91.7%           
=========================================
  Files             ?      150           
  Lines             ?    49152           
  Branches          ?        0           
=========================================
  Hits              ?    45074           
  Misses            ?     4078           
  Partials          ?        0

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

Impacted Files	Coverage Δ
pandas/core/indexes/base.py	96.66% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d7bcb22...e6aad42. Read the comment docs.

[jorisvandenbossche]

@Donk23 made a minor edit, thanks a lot!