Index: llvm/docs/CodingStandards.rst
===================================================================
--- llvm/docs/CodingStandards.rst
+++ llvm/docs/CodingStandards.rst
@@ -305,6 +305,21 @@
 #. When writing a source file that is used by a tool that only accepts C style
    comments.
 
+#. To document the significance of constants used as actual parameters in a call.
+   This is most helpful for ``bool`` parameters, or passing ``0`` or ``nullptr``.
+   Typically you add the formal parameter name, which ought to be meaningful.
+   For example, it's not clear what the parameter means in this call:
+
+  .. code-block:: c++
+
+    Object.emitName(nullptr);
+
+  An in-line C-style comment makes the intent obvious:
+
+  .. code-block:: c++
+
+    Object.emitName(/*Prefix=*/nullptr);
+
 Commenting out large blocks of code is discouraged, but if you really have to do
 this (for documentation purposes or as a suggestion for debug printing), use
 ``#if 0`` and ``#endif``. These nest properly and are better behaved in general