Clarify what "dispose" means for the runtime

[picklesrus]

The purpose of Runtime's dispose isn't well defined.

Here are a few examples, though there are likely more:

• It doesn't stop the stepping loop for threads, (is this an oversight or intentional?)
• It only clears some monitor blocks because of toolbox block recycling (See discussion in Clear out the blocks in dispose. Fixes #1758 where old monitored vari… #1818)