[LLVM][DOCS] Add documentation for 'host' and 'Native' options in LLV…

…M_TARGETS_TO_BUILD. (llvm#111382)

From llvm#111356

llvm/docs/CMake.rst

@@ -847,6 +847,12 @@ 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
+  include the target corresponding to the host machine's architecture, or
+  use ``all`` to include all available targets.
+  For example, on an x86_64 machine, specifying ``-DLLVM_TARGETS_TO_BUILD=host``
+  will include the ``X86`` target.
+
 **LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN**:BOOL
   If enabled, the compiler version check will only warn when using a toolchain
   which is about to be deprecated, instead of emitting an error.