docs: add the docs for loc and iloc indexers

[ashleyxuu]

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Fixes #<issue_number_goes_here> 🦕

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[tswast]

Please also update the iloc, loc docstrings in

python-bigquery-dataframes/third_party/bigframes_vendored/pandas/core/frame.py

Line 5113 in 91bd39e

def iloc(self):

and

python-bigquery-dataframes/third_party/bigframes_vendored/pandas/core/series.py

Line 3315 in 91bd39e

def iloc(self):

to point to the corresponding indexer class in the Returns: section.

[tswast]

Thanks!