Document safe getter value descriptors.
Add 'enumerable' and 'writable' props to safe getter value descriptors.
Fix typo in safeGetterValues example.
Tweak description of 'sources' request and source actors.
Document Debugger.Script.prototype.sourceMapURL.
Spell out effect of duplicate debuggee adds.
Add [options] to the four "eval" methods. Corresponds to changes from bug 889627.