Details that -debug-only is not available when LLVM is built with --enable-optimized

llvm-svn: 218447
diff --git a/llvm/docs/ProgrammersManual.rst b/llvm/docs/ProgrammersManual.rst
index b06d2eb..a3a20f3 100644
--- a/llvm/docs/ProgrammersManual.rst
+++ b/llvm/docs/ProgrammersManual.rst
@@ -425,6 +425,8 @@
 instruction scheduling to be enabled with ``-debug-only=InstrSched``, even if
 the source lives in multiple files.
 
+For performance reasons, -debug-only is not available in optimized build (``--enable-optimized``) of LLVM.
+
 The ``DEBUG_WITH_TYPE`` macro is also available for situations where you would
 like to set ``DEBUG_TYPE``, but only for one specific ``DEBUG`` statement.  It
 takes an additional first parameter, which is the type to use.  For example, the