Spellchecker requires exceptions for plurals and possessives

As per title. Valid words that otherwise are accepted by the spellchecker are being flagged as errors in their plural and possessive forms (mostly acronyms, but I've been seeing it with other words as well).

I'm currently performing an audit of my exception list, once it's landed I can link it here to show the words being flagged as errors that the spellchecker should be able to understand.

The issue names no file or test; start with the exception-list audit described in the report and collect representative plural and possessive forms that are flagged. Done means the audited valid forms are accepted by the spellchecker without inappropriate exceptions.

Written by the indexing model from the issue text.