[DOC] Asserts are only enabled in Debug build, update the doc accordingly

[mehdi_amini]

Summary

The CMake configuration is explicitely looking for Debug build, all the
other variant disable assertions.

[mehdi_amini]

If anyone think that assertions should be enabled by default for Release, RelWithDebInfo or MinSizeRel, please speak up before I update the doc :)

docs/CMake.rst

 
 **LLVM_ENABLE_THREADS**:BOOL
   Build with threads support, if available. Defaults to ON.
 
 **LLVM_ENABLE_CXX1Y**:BOOL
   Build in C++1y mode, if available. Defaults to OFF.
 
 **LLVM_ENABLE_ASSERTIONS**:BOOL
-  Enables code assertions. Defaults to OFF if and only if ``CMAKE_BUILD_TYPE``
-  is *Release*.
+  Enables code assertions. Defaults to ON if and only if ``CMAKE_BUILD_TYPE``
+  is *Debug*.
 
 **LLVM_ENABLE_EH**:BOOL
   Build LLVM with exception handling support. This is necessary if you wish to
   link against LLVM libraries and make use of C++ exceptions in your own code
   that need to propagate through LLVM code. Defaults to OFF.
 
 **LLVM_ENABLE_PIC**:BOOL
   Add the ``-fPIC`` flag for the compiler command-line, if the compiler supports
**LLVM_ENABLE_DOXYGEN_QT_HELP**:BOOL
   Enables the generation of a Qt Compressed Help file. Defaults to OFF.
   This affects the make target ``doxygen-llvm``. When enabled, apart from
   the normal HTML output generated by doxygen, this will produce a QCH file
   named ``org.llvm.qch``. You can then load this file into Qt Creator.
   This option is only useful in combination with ``-DLLVM_ENABLE_DOXYGEN=ON``;
   otherwise this has no effect.
 
 **LLVM_DOXYGEN_QCH_FILENAME**:STRING
-  The filename of the Qt Compressed Help file that will be genrated when
-  ``-DLLVM_ENABLE_DOXYGEN=ON`` and 
+  The filename of the Qt Compressed Help file that will be generated when
+  ``-DLLVM_ENABLE_DOXYGEN=ON`` and
   ``-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON`` are given. Defaults to
   ``org.llvm.qch``.
   This option is only useful in combination with
   ``-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON``;
   otherwise this has no effect.
 
 **LLVM_DOXYGEN_QHP_NAMESPACE**:STRING
   Namespace under which the intermediate Qt Help Project file lives. See `Qt
   Help Project`_
   for more information. Defaults to "org.llvm". This option is only useful in
   combination with ``-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON``; otherwise
   this has no effect.
-    
 **LLVM_DOXYGEN_QHP_CUST_FILTER_NAME**:STRING
   See `Qt Help Project`_ for
   more information. Defaults to the CMake variable ``${PACKAGE_STRING}`` which
   is a combination of the package name and version string. This filter can then
   be used in Qt Creator to select only documentation from LLVM when browsing
   through all the help files that you might have loaded. This option is only
   useful in combination with ``-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON``;
   otherwise this has no effect.