[gcc r13-2675] libstdc++: Add comment to 17_intro/names.cc test

[Jonathan Wakely]

https://gcc.gnu.org/g:9a1bc4b463e61cd5bc5f169a08a420aae39a07f6

commit r13-2675-g9a1bc4b463e61cd5bc5f169a08a420aae39a07f6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 14 14:16:25 2022 +0100

    libstdc++: Add comment to 17_intro/names.cc test
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/17_intro/names.cc: Explain why poison pragma can't
            be used.

Diff:
---
 libstdc++-v3/testsuite/17_intro/names.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/testsuite/17_intro/names.cc b/libstdc++-v3/testsuite/17_intro/names.cc
index 86fb8f8999b..82e201c71b0 100644
--- a/libstdc++-v3/testsuite/17_intro/names.cc
+++ b/libstdc++-v3/testsuite/17_intro/names.cc
@@ -20,6 +20,8 @@
 
 // Define macros for some common variables names that we must not use for
 // naming variables, parameters etc. in the library.
+// N.B. we cannot use '#pragma GCC poison A' because that also prevents using
+// these names even as macro arguments, e.g. #define FOO(A) BAR(A)
 #define A (
 #define B (
 #define C (