DOC: Calrify tiny/xmin in finfo and machar (gh-16253)

This commit adds a note to the documentation of finfo clarifying that
the attribute `tiny` does not represent the smallest positive usable
number. Instead it refers to the smallest positive number with there
being no leading 0's in the mantisse (normalized floating point number).
Alternatively, `tiny` represents the smallest positive floating point
number with full precision. The smallest positive subnormal floating
point number is typically orders of magnitudes smaller, but has reduced
precision since leading 0's in the mantisse are used to allow smaller
exponents than indicated by `minexp`.

The commit also adds a brief clarification to the docstring of machar.

Closes #16252

Co-authored-by: Anirudh Subramanian <anirudh2290@apache.org>
Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

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