cubicweb #3360868 document the various <something>_(R)TYPES variables [open]

As of today one can find:

META_RTYPES
VIRTUAL_RTYPES
PURE_VIRTUAL_RTYPES
READ_ONLY_RTYPES
SYSTEM_RTYPES

BASE_TYPES
INTERNAL_TYPES
SCHEMA_TYPES
SYSTEM_ENTITIES
SOURCE_TYPES

and probably some more ....

From their names & usage alone it is not always easy to find what to use when needed.

prioritynormal
typeenhancement
done in<not specified>
load0.500
load left0.500
closed by<not specified>