Clarify the effect of changing problems

@explorerleslie , I tried to place the warnings we discussed carefully, and sharpen existing passages to clarify the cons of making changes.
@WatsonEmily , if PMs have thoughts about this wording (how frequently to include warnings in the doc, how severe the tone should be, if there should be more examples of when adjustments might be needed or even fewer, etc) I would love the input.
@stroilova , since the JIRA item was filed, the user interface for Studio has been enhanced to include better indicators and warnings when live courseware is changed (including ORA2 problems). 
@mhoeber , please copyedit.
not just grading policy -- but changes to a graded component, right?