The current documentation on Creating Issues and Comments from Email : Handler Parameters says:

ccassignee
If an email has a Cc address listing a user already present in JIRA, by default JIRA will assign the issue created from the email to that user. If you do not want this behaviour, set ccassignee to false.

The name ccassignee is misleading as it indicates a relationship with email's CC field, however JIRA uses all fields (To, Cc, Bcc) when performing a lookup for an assignable user. Therefore the message should say something along the following lines:

If an email's To, Cc, or Bcc field (in this order) has any address listing a user already present in JIRA, by default JIRA will assign the issue created from the email to that user. If you do not want this behaviour, set ccassignee to false.