Fix documentation for TEveProjected::GetProjectedAsElement(), see ROOT-6246.

(cherry picked from v5-34-00-patches commit 746a973aaf828c2be57b97336ee9c1f21ff29199)
Document the fact that ~TEveElement should not be invoked directly, see ROOT-6247.

(cherry picked from v5-34-00-patches commit 0c5e70a2e44cb9f0370cb936ad84824645f92761)