New ``min_digits`` argument for printing float values
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A new ``min_digits`` argument has been added to the dragon4 float printing
functions `np.format_float_positional` and `np.format_float_scientific` . This
kwd guarantees that at least the given number of digits will be printed when
printing in unique=True mode, even if the extra digits are unnecessary to
uniquely specify the value. It is the counterpart to the precision argument
which sets the maximum number of digits to be printed. When unique=False in
fixed precision mode, it has no effect and the precision argument fixes the
number of digits.

Add release note for gh-18629.
DOC: Add release note for gh-18629.
