box: Add documentation for `From` impls#55470

Merged

bors merged 4 commits intorust-lang:masterfrom

daniellimws:box-from-docs

Dec 24, 2018

Commits on Oct 29, 2018

Commits on Nov 16, 2018

Add double quotes to resolve error
daniellimws
committed

Commits on Dec 5, 2018

Fix error in example by adding type annotation
daniellimws
committed