Jira online documentation is incorrect in regard to the behaviour of JQL when searching for issue not assigned to a given user. Since the official position from Atlassian on the topic of equality being broken for unassigned issues is that it will not change then the online documentation should reflect correct usage.

The Jira documentation on currentUser https://confluence.atlassian.com/display/JIRA/Advanced+Searching+Functions#AdvancedSearchingFunctions-currentUser%28%29 provides a JQL example that will "find issues that were reported to [sic] me but are not assigned to me"

reporter = currentUser() and assignee != currentUser()

however this will not return issues that are unassigned.

It is likely the example JQL should be :

reporter = currentUser() AND (assignee != currentUser() OR assignee is EMPTY)

Personally, I would prefer it if JQL were fixed but failing that the docs should be updated.

I would encourage you to update docs with a description of "correct" usage of JQL where a field is empty as it is counter intuitive that logical operators should behave this way.