core: Improve docs for cell
std: Fix broken link
core: More concise description for mod ops
core: Spruce up the crate description
std: Alphabetize crate reexports for rustdoc
core: Convert TODOs to FIXMEs
Address review comments
auto merge of #14304 : brson/rust/moredocs, r=kballard

Includes module docs for `cell`.