Clarify majority/consensus criteria

[dwijnand]

As of #188, the current FCP proposed comment says "Once a majority of reviewers approve (and none object)" and the conditions are num_active_concerns == 0 && num_outstanding_reviews < num_complete_reviews && num_outstanding_reviews < 3.

In an proposal in cargo, the contributor thought it was strict majority: rust-lang/cargo#6535 (comment).

Could the messaging be improved?

[jethrogb]

Looks like the common case is "Once all but 2 reviewers approve". Only if the reviewer set size is 4 or less is it "Once a majority of reviewers approve". (Either statement is correct for sizes 5-6).