Fix docstrings for int/u64, int/s64 and int/to-number

[cideM]

All credits go to @sogaiu (see this Zulip chat).

The current doc strings for int/u64 and int/s64 say that these functions only support strings. But as far as I can tell from the diff they always support both numbers and strings, and the test suites cover those cases. I therefore updated the docstrings to indicate that.

int/to-number used to only supports int32 values but this commit added support for up to JANET_INTMAX_INT64.

This isn't the most ground breaking commit but I was certainly a bit confused about what I can do with these functions when I was dealing with bxoring 64bit integers for Advent of Code.

[sogaiu]

As an additional data point, it looks like:

(int/to-number (+ (int/u64 (math/pow 2 53)) 1))

leads to an error currently:

error: cannot convert <core/u64 9007199254740993> to a number, must be in the range [-9007199254740992, 9007199254740992]
  in int/to-number [src/core/inttypes.c] on line 206
  in thunk [repl] (tail call) on line 19, column 1

FWIW, one less seems to work:

(int/to-number (int/u64 (math/pow 2 53)))
# =>
9007199254740992