Fix description of stretchable lengths

This previously seemed wrong both in the syntax (two `minus`es rather than one `plus` and one `minus`) and in the mathematical inequality, which I've corrected to match my formulation of the syntax; but both could be done differently.
Merge pull request #201 from rrthomas/master

Fix description of stretchable lengths
Merge branch 'master' into 1.12.5-1.8.3