Added comments to public functions in num.rs.

Copied existing comments from other files, and added comments to
functions that did not have existing comments available.
Addressing huonw's comments.
Addressing cmr's comments.
auto merge of #9585 : vky/rust/num-docs, r=brson

Copied existing comments from other files, and added comments to functions that did not have existing comments available. 

cc #7511