Add some missing method wrappers to std::num
Improve std::num module description, and fix some formatting
FALSE is not 1u8 but 0u8, of cource.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #8137 : omasanori/rust/ubool-false, r=bblum

Obvious mistake, but there is no failure because no code uses FALSE.
auto merge of #8115 : bjz/rust/num-traits, r=brson

Continues #4819