Refine description of Descriptors in ToC
Changelog documents Frame --> QFrame rename
API documentation tweaks
QElement.md tweaks
QElementList.md tweaks
descriptors.md tweaks
Additional descriptors.md tweaks
INTEGRATE: 0.5 documentation tweaks

* dev:
  Additional descriptors.md tweaks
  descriptors.md tweaks
  QElementList.md tweaks
  QElement.md tweaks
  API documentation tweaks
  Changelog documents Frame --> QFrame rename
  Refine description of Descriptors in ToC
0.5.0