[docs] [issue8952] Doc/c-api/arg.rst: fix documentation of number formats

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

Yes, most of that paragraph is outdated.

We should check exactly what does happen when the "receiving field is too small" (both in practice and in theory).  In C, downcasting to an unsigned type is well-defined and will always reduce modulo 2**<width_of_type>.  The result of downcasting to a signed type is implementation-defined, however;  do we actually do that anywhere?  If so, it would be nice to fix it.

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