Clarify behaviour of the 'Remove Reviewer' permission for change owners

Without having the 'Remove Reviewer' permission, change owners can only
remove reviewers that have given a zero or positive score.  I.e. they
cannot remove reviewers that have given negative scores.

Clarify this in the documentation.

Change-Id: Ic1c5a292607f64337f82596553066cdbcd04eda2