Environment variable GHC_CHARENC lacks documentation

Location of documentation issue: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/using.html#other-environment-variables

GHC_CHARENC is undocumented.

GHC_CHARENC is used e.g. here: https://github.com/ghc/ghc/blob/0dc723957d0fdb5909f145405b775efea0fe2f6e/libraries/ghc-boot/GHC/HandleEncoding.hs#L9-L17

Blame goes to https://github.com/ghc/ghc/commit/1b56c40578374a15b4a2593895710c68b0e2a717

Context: https://github.com/agda/agda/issues/5742

Btw: GHC_CHARENC does not follow any identifier standard I am aware of. (As opposed to GHC_CHAR_ENC or GHC_CHAR_ENCODING.) [Also, my subconscious produces misspellings such as GHC_CHARENV and GHC_CODEENV.]

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