Reword and clarify the doc strings for digits in the analog payload

Reword and clarify the doc strings for digits in the analog payload

Clarifies the intended use of the properties sr_analog_encoding.digits and
sr_analog_spec.spec_digits used in the analog payload and also digits() in
the Analog class of the C++ bindings.

These properties hold the number of significant digits after the decimal
point, if positive. When negative, exponent with reversed polarity that is
necessary to express the value with all digits without a decimal point.