[diff.mods.to.definitions] Add entry for 'nullptr_t in stddef.h'

Author: tkoeppe

source/compatibility.tex

@@ -2249,6 +2249,16 @@
 They do not appear as macro names defined in
 \tcode{<cstdbool>}\indexlibrary{\idxhdr{cstdbool}}~(\ref{cstdbool.syn}).
 
+\rSec3[diff.header.stddef.h]{Header \tcode{<stddef.h>}}
+\indexlibrary{\idxhdr{stddef.h}}%
+
+\pnum
+The headers \tcode{<cstddef>}\indexlibrary{\idxhdr{cstddef}}~(\ref{cstddef.syn})
+and \tcode{<stddef.h>}\indexlibrary{\idxhdr{stddef.h}} declare the name
+\tcode{nullptr_t} (respectively in namespace \tcode{std} and in the
+global namespace) in addition to the names declared in \tcode{<stddef.h>}
+in the C standard library.
+
 \rSec3[diff.null]{Macro \tcode{NULL}}
 
 \pnum