DOC: list supported float dtypes I
- [x] closes #50216
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Updated documentation for `pandas.to_numeric()`, pointed out that `downcast` parameter doesn’t support `float16` dtype. Listed in the description of [dtype](https://pandas.pydata.org/docs/user_guide/basics.html#dtypes) supported floating point types and added Link to [NumPy types](https://numpy.org/doc/stable/reference/arrays.scalars.html#numpy).
thx @natmokval 