readlink.2: Glibc falls back to readlink() on kernels that lack readlinkat()

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/readlink.2

@@ -253,6 +253,19 @@ also addresses a common portability problem when using
 for the buffer size,
 as this constant is not guaranteed to be defined per POSIX
 if the system does not have such limit.
+.SS Glibc notes
+On older kernels where
+.BR readlinkat ()
+is unavailable, the glibc wrapper function falls back to the use of
+.BR readlink ().
+When
+.I pathname
+is a relative pathname,
+glibc constructs a pathname based on the symbolic link in
+.IR /proc/self/fd
+that corresponds to the
+.IR dirfd
+argument.
 .SH EXAMPLE
 The following program allocates the buffer needed by
 .BR readlink ()