Explain _POSIX_VERSION >= 200112L test for fseeko

diff --git a/src/io.c b/src/io.c
index 823be1a..08eb032 100644
--- a/src/io.c
+++ b/src/io.c

@@ -42,7 +42,9 @@
 #define AVIF_USE_FSEEKO
 #endif
 #elif defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
-/* Standard Modern POSIX */
+// Standard Modern POSIX. The _POSIX_VERSION >= 200112L test for fseeko/ftello
+// is used in the first example in the APPLICATION USAGE section in
+// https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/unistd.h.html.
 #define AVIF_USE_FSEEKO
 #endif