[docs] Rewrite/improve the docs for LLVM_NATIVE_TOOL_DIR

[mstorsjo]

Summary

Don't include it among the mandatory options; the automatically built
tools via a nested cmake build work fine these days
(in particular, since 93010544a813dfbfa64dd7cee68785f572f974d1 /
https://reviews.llvm.org/D126313).

Clarify the directory path-to-host-bin into something more verbose,
to avoid ambiguity with LLVM_HOST_TRIPLE.

This goes on top of D142404.

[peter.smith]

No objections, if the option isn't needed then we should remove it from the mandatory part.

[phosek]

LGTM

llvm/docs/HowToCrossCompileLLVM.rst

 
 For more information on how to configure CMake for LLVM/Clang,
 see :doc:`CMake`.
 
 The CMake options you need to add are:
 
  * ``-DCMAKE_SYSTEM_NAME=<target-system>``
  * ``-DCMAKE_INSTALL_PREFIX=<install-dir>``
- * ``-DLLVM_NATIVE_TOOL_DIR=<path-to-host-bin>``
  * ``-DLLVM_HOST_TRIPLE=arm-linux-gnueabihf``
  * ``-DLLVM_TARGETS_TO_BUILD=ARM``
 
 Note: ``CMAKE_CROSSCOMPILING`` is always set automatically when ``CMAKE_SYSTEM_NAME`` is set. Don't put ``-DCMAKE_CROSSCOMPILING=TRUE`` in your options.
 
 Also note that ``LLVM_HOST_TRIPLE`` specifies the triple of the system
 that the cross built LLVM is going to run on - the flag is named based
 on the autoconf build/host/target nomenclature. (This flag implicitly sets

 In addition to the ones above, you'll also need:
 
  * ``--target=arm-linux-gnueabihf`` or whatever is the triple of your cross GCC.
  * ``'--sysroot=/usr/arm-linux-gnueabihf'``, ``'--sysroot=/opt/gcc/arm-linux-gnueabihf'``
    or whatever is the location of your GCC's sysroot (where /lib, /bin etc are).
  * Appropriate use of ``-I`` and ``-L``, depending on how the cross GCC is installed,
    and where are the libraries and headers.
 
-The ``LLVM_NATIVE_TOOL_DIR`` option allows you to reuse prebuilt binaries
-(``llvm-tblgen``, ``clang-tblgen`` etc) for the build host, if such are
-available. If that's not available, the LLVM cross build will automatically
-launch a nested build to build the tools that are required.
+You may also want to set the ``LLVM_NATIVE_TOOL_DIR`` option - pointing
+at a directory with prebuilt LLVM tools (``llvm-tblgen``, ``clang-tblgen``
+etc) for the build host, allowing you to them reuse them if available.
+E.g. ``-DLLVM_NATIVE_TOOL_DIR=<path-to-native-llvm-build>/bin``.
+If the option isn't set (or the directory doesn't contain all needed tools),
+the LLVM cross build will automatically launch a nested build to build the
+tools that are required.
 
 The CXX flags define the target, cpu (which in this case
 defaults to ``fpu=VFP3`` with NEON), and forcing the hard-float ABI. If you're
 using Clang as a cross-compiler, you will *also* have to set ``--sysroot``
 to make sure it picks the correct linker.
 
 When using Clang, it's important that you choose the triple to be *identical*
 to the GCC triple and the sysroot. This will make it easier for Clang to