document what unsafety means

Closes #9144
Generate random big integers within a range

Thanks to @huonw for feedback
auto merge of #9258 : thestinger/rust/doc, r=catamorphism

Closes #9144
auto merge of #9133 : dcrewi/rust/bigint-random-range, r=huonw
std: Fix an invalid read in from_c_multistring

When `count` is `Some` this function was reading a byte past the end
of the buffer.
auto merge of #9281 : brson/rust/multistring, r=alexcrichton

When `count` is `Some` this function was reading a byte past the end
of the buffer.