modify queue implementation documentation

This commit only has documentation changes and a few things moved around
the file. The very few code changes are cosmetic: changes like turning a
`match` statement into an `if let` statement or reducing indentation for
long if statements.

This commit also adds several safety comments on top of `unsafe` blocks
that might not be immediately obvious to a first-time reader.

Code "changes" are in:
- `add_backlinks_and_find_tail`
- `lock_contended`

A majority of the changes are just expanding the comments from 80
columns to 100 columns.