Add documentation for GHC-21926

[FPtje]

This is the card I drew during ZuriHac! :card_index:

This is quite a simple parser error, which can be hit when PackageImports is enabled. The error itself is quite clear already: don't put version numbers and non-alphanumeric characters in a package import.

I'm not sure how one would get into the non-alphanumeric character situation (besides an honest typo), but the version one is quite a fair mistake to make.