[llvm] [LLVM][DOCS] Add documentation for 'host' and 'Native' options in LLVM_TARGETS_TO_BUILD. (PR #111382)

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PerMildner wrote:

Not mentioned in the issue, but the value `all` is apparently also one of the "symbolic" values, so perhaps that too should be explicitly mentioned.

https://github.com/llvm/llvm-project/pull/111382