Document the behavior of option passing when using -DCLANG_ENABLE_BOOTSTRAP=On Also document -DCLANG_BOOTSTRAP_PASSTHROUGH

[sylvestre.ledru]

Fix a typo

Updating D53018: Document the behavior of option passing when using -DCLANG_ENABLE_BOOTSTRAP=On

Also document -DCLANG_BOOTSTRAP_PASSTHROUGH

docs/AdvancedBuilds.rst

.. code-block:: console
 
   $ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On <path to source>
   $ ninja stage2
 
 This command itself isn't terribly useful because it assumes default
 configurations for each stage. The next series of examples utilize CMake cache
 scripts to provide more complex options.
 
+By default, only a few CMake options will be passed between stages.
+The list, called _BOOTSTRAP_DEFAULT_PASSTHROUGH, in defined in clang/CMakeLists.txt.
+To force the passing of the variables between stages, use the -DCLANG_BOOTSTRAP_PASSTHROUGH
+CMake option, each variable separated by a ";". As example:
+
+.. code-block:: console
+
+  $ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" <path to source>
+  $ ninja stage2
+
 The clang build system refers to builds as stages. A stage1 build is a standard
 build using the compiler installed on the host, and a stage2 build is built
 using the stage1 compiler. This nomenclature holds up to more stages too. In
 general a stage*n* build is built using the output from stage*n-1*.
 
 Apple Clang Builds (A More Complex Bootstrap)
 =============================================