ThinLTO: Document the option BOOTSTRAP_LLVM_ENABLE_LTO

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370671 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ThinLTO.rst

@@ -225,6 +225,10 @@ To bootstrap clang/LLVM with ThinLTO, follow these steps:
    ``CMAKE_EXE_LINKER_FLAGS:STRING=``. Note the configure may fail if
    linker plugin options are instead specified directly in the previous step.
 
+The `BOOTSTRAP_LLVM_ENABLE_LTO=Thin`` will enable ThinLTO for stage 2 and
+stage 3 in case the compiler used for stage 1 does not support the ThinLTO
+option.
+
 More Information
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