[Rd] R-specific environment variables: Naming convention?

[Henrik Bengtsson]

In base R, there are lots of environment variables with either prefix
'R_' or '_R_', e.g. R_ENABLE_JIT and _R_RNG_VERSION_.  I always
considered R_* variables to be "public" and _R_*_ ones being
"internal" but realized I don't have a reference for this.  Is this
true, or is there another reason? Is the difference between the two
kinds documented anywhere?

Thank you,

Henrik