Add the description about custom resource classes and
overriding standard resource classes in the "Flavors" document.
Conflicts:
doc/source/user/flavors.rst
Due to: The following changes are not backported.
* I9bd80adc244c64277d2d00e7d79c3002c8f9d57e
* Ifb99e727110c4904a85bc4a13366c2cae300b8df
Change-Id: I5b804db70d229696e7b7c5b5db16946cf1f1c49f
Closes-Bug: #1800663
(cherry picked from commit 0e718ddb7a)
(cherry picked from commit e54c5fbe2d)