Help: Correct CUDAHOSTCXX and CUDAARCHS as having higher precedence

For CUDAHOSTCXX the behaviour seems to have been like this since the
introduction of it in commit 9cf5b98d ("CUDA: Prefer environment variables
CUDACXX and CUDAHOSTCXX.", 2016-11-08) and is likely unintentional judging by
the wording of the commit.

The documentation mistake seems to be a copy-paste error from when all the
environment variables were documented in commit e6b77c5f ("Help: Document
CMake's environment variables", 2017-09-01).
Describe this explicitly as it is unlike all other similar environment
variables.

For CUDAARCHS we got it wrong from the get-go in commit c4ae9384 ("CUDA:
Initialize CMAKE_CUDA_ARCHITECTURES using $ENV{CUDAARCHS}", 2020-11-24).

Correcting either to follow the more standard precedence behaviour will require
a policy.

Fixes #23081.

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