Note that Z3_get_numeral_small is essentially redundant

[jberdine]

The error behavior of Z3_get_numeral_small does not follow the pattern of
the other functions. The functions that have out params and return a bool
indicating success (such as Z3_get_numeral_rational_int64) return false
rather than signaling an error when given an unsupported expression
argument (such as a rounding mode value). The functions that do not have out
params signal an error in such cases. Z3_get_numeral_small is the odd one
out in that it signals errors and returns a status bool.

This error handling is the only difference between Z3_get_numeral_small and
Z3_get_numeral_rational_int64, so this patch adds a comment to the
documentation.

Also:
Check that Z3_get_numeral_small is given non-null out params

Analogous to other Z3_get_numeral_* functions with out params.