[extern.types] Use "declared" for C library types

Author: frederick-vs-ja

source/lib-intro.tex

@@ -3384,7 +3384,7 @@
 \tcode{::T}
 and
 \tcode{std::T}
-are reserved to the implementation and, when defined,
+are reserved to the implementation and, when declared,
 \tcode{::T}
 shall be identical to
 \tcode{std::T}.