Improve CARGO_ENCODED_RUSTFLAGS documentation

[ehuss]

CARGO_ENCODED_RUSTFLAGS and CARGO_ENCODED_RUSTDOCFLAGS were added in #9601. The current documentation may be a little confusing. I think the following changes could help:

• In Environment variables Cargo reads, include both environment variables as dedicated entries to the list.
• I would also reword the existing "for a more robust encoding of multiple arguments, set CARGO_ENCODED_RUSTFLAGS instead with arguments separated by 0x1f (ASCII Unit Separator)." to be "for a more robust encoding of multiple arguments, see CARGO_ENCODED_RUSTFLAGS".
• Update build.rustflags and build.rustdocflags to include the new environment variables, and to document the precedence behavior.

[0xPoe]

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