random_r.3: Note that 'buf' records a pointer to 'statebuf'

See https://sourceware.org/bugzilla/show_bug.cgi?id=3662.

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

man3/random_r.3

@@ -97,6 +97,23 @@ rather than initializing the global state variable.
 Before calling this function, the
 .IR buf.state
 field must be initialized to NULL.
+The
+.BR initstate_r ()
+function records a pointer to the
+.I statebuf
+argument inside the structure pointed to by
+.IR buf .
+Thus,
+.IR statebuf
+should not be deallocated so long as
+.IR buf
+is still in use.
+(So,
+.I statebuf
+should typically be allocated as a static variable,
+or allocated on the heap using
+.BR malloc (3)
+or similar.)
 
 The
 .BR setstate_r ()