Update API docs for KDbDriverManager::driverIdsForMimeType()

• IDs of drivers for file-based databases are only returned.
• Empty list is returned if no driver has been found for the type or if the type is invalid.
• Driver supports the supplied MIME type if it is specified as supported in the driver's
• metadata. If a MIME type alias is supplied, proper type for this alias is resolved and driver
• IDs for that type are returned. Similarly, if proper MIME type is supplied, IDs are returned
• for drivers that support any alias for this type.

+ Restore original logic: the MIME type lookup is case insensitive