Update defining-blocks.md
Update defining-blocks.md

add section about min and max values.
playi/pxt,Microsoft/pxt,switchinnovations/pxt,switchinnovations/pxt,switch-education/pxt,playi/pxt,playi/pxt,Microsoft/pxt,switchinnovations/pxt,playi/pxt,Microsoft/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt,Microsoft/pxt