DOC: Clarify pivot_table fill_value description
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

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Clarify that `fill_value` gets used after performing the aggregation (i.e. it fills values in the resulting pivot table).
(Depending on the aggfunc and df, the result will be different if performing the filling before/after aggregation).

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### Actually, why not remove it, requiring the user to more explicitly do `df.pivot_table(...).fillna(...)`?