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

[Fangrui Song via llvm-commits]

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@@ -836,6 +836,12 @@ enabled sub-projects. Nearly all of these variable names begin with
   The full list, as of March 2023, is:
   ``AArch64;AMDGPU;ARM;AVR;BPF;Hexagon;Lanai;LoongArch;Mips;MSP430;NVPTX;PowerPC;RISCV;Sparc;SystemZ;VE;WebAssembly;X86;XCore``
 
+  You can also specify ``host`` or ``Native`` to automatically detect and
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MaskRay wrote:

Just document the common `host` and omit `Native`?

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