[docs] Expand example on stand-alone builds.

[fpetrogalli]

Summary

1. Make explicit that the folder where to build a subproject in stand-alone mode can not be the same folder where LLVM was build.
2. Add a cut 'n paste example for building stand-alone clang.

[fpetrogalli]

s/standalone/stand-alone/

[tstellar]

LGTM

[fpetrogalli]

Thank you @tstellar

llvm/docs/GettingStarted.rst

 
 For stand-alone builds, you must have an llvm install that is configured
 properly to be consumable by stand-alone builds of the other projects.
 This could be a distro provided LLVM install, or you can build it yourself,
 like this:
 
 .. code-block:: console
 
-  cmake -G Ninja -S llvm -B $builddir \
+  cmake -G Ninja -S path/to/llvm-project/llvm -B $builddir \
         -DLLVM_INSTALL_UTILS=ON \
         -DCMAKE_INSTALL_PREFIX=/path/to/llvm/install/prefix \
         < other options >
 
   ninja -C $builddir install
 
 Once llvm is installed, to configure a project for a stand-alone build, invoke CMake like this:
 
 .. code-block:: console
 
-  cmake -G Ninja -S $subproj -B $buildir \
+  cmake -G Ninja -S path/to/llvm-project/$subproj \
+        -B $buildir_subproj \
         -DLLVM_EXTERNAL_LIT=/path/to/lit \
         -DLLVM_ROOT=/path/to/llvm/install/prefix
 
-``LLVM_ROOT`` should point to the prefix of your llvm installation, so for example,
-if llvm is installed into ``/usr/bin`` and ``/usr/lib64``, then you should pass
-``-DLLVM_ROOT=/usr/``.  Both the ``LLVM_ROOT`` and ``LLVM_EXTERNAL_LIT`` options
-are required to do stand-alone builds for all sub-projects.  Additional required
-options for each sub-project can be found in the table below.
+Notice that:
+
+* The stand-alone build needs to happen in a folder that is not the
+  original folder where LLVMN was built
+  (`$builddir!=$builddir_subproj`).
+* ``LLVM_ROOT`` should point to the prefix of your llvm installation,
+   so for example, if llvm is installed into ``/usr/bin`` and
+   ``/usr/lib64``, then you should pass ``-DLLVM_ROOT=/usr/``.
+* Both the ``LLVM_ROOT`` and ``LLVM_EXTERNAL_LIT`` options are
+  required to do stand-alone builds for all sub-projects.  Additional
+  required options for each sub-project can be found in the table
+  below.
 
 The ``check-$subproj`` and ``install`` build targets are supported for the
 sub-projects listed in the table below.
 
 ============ ======================== ======================
 Sub-Project  Required Sub-Directories Required CMake Options
 ============ ======================== ======================
 llvm         llvm, cmake, third-party LLVM_INSTALL_UTILS=ON
 clang        clang, cmake             CLANG_INCLUDE_TESTS=ON (Required for check-clang only)
 lld          lld, cmake
 ============ ======================== ======================
 
+Example for building stand-alone `clang`:
+
+.. code-block:: console
+
+   #!/bin/sh
+
+   build_llvm=`pwd`/build-llvm
+   build_clang=`pwd`/build-clang
+   installprefix=`pwd`/install
+   llvm=`pwd`/llvm-project
+   mkdir -p $build_llvm
+   mkdir -p $installprefix
+
+   cmake -G Ninja -S $llvm/llvm -B $build_llvm \
+         -DLLVM_INSTALL_UTILS=ON \
+         -DCMAKE_INSTALL_PREFIX=$installprefix \
+         -DCMAKE_BUILD_TYPE=Release
+
+   ninja -C $build_llvm install
+
+   cmake -G Ninja -S $llvm/clang -B $build_clang \
+         -DLLVM_EXTERNAL_LIT=$build_llvm/utils/lit \
+         -DLLVM_ROOT=$installprefix
+
+   ninja -C $build_clang
 
 Requirements
 ============
 
 Before you begin to use the LLVM system, review the requirements given below.
 This may save you some trouble by knowing ahead of time what hardware and
 software you will need.