The documentation for 3.2.1 has been archived at:
There's an article for upgrading from 3.1.2 to 4 on the Github wiki for the Font Awesome project:
You can also download earlier releases from the project's Releases page. The 3.2.1 release does include docs, but they must be compiled to be useful.
Generally speaking, you can always check the Wayback Machine as a last resort, which has cached versions of some of the docs from previous releases based on date: