[cpp.replace] Adjust footnote to clarify that conditionally-supported-directives are directives whether supported or not (#1045)

Author: timsong-cpp

source/preprocessor.tex

@@ -732,7 +732,7 @@
 but comma preprocessing tokens between matching
 inner parentheses do not separate arguments.
 If there are sequences of preprocessing tokens within the list of
-arguments that would otherwise act as preprocessing directives,\footnote{Despite the name, a non-directive is a preprocessing directive.}
+arguments that would otherwise act as preprocessing directives,\footnote{A \grammarterm{conditionally-supported-directive} is a preprocessing directive regardless of whether the implementation supports it.}
 the behavior is undefined.
 
 \pnum