rtld: explain the use of rtld_bind_lock in rtld_get_addr_slow()

Discussed with:	jrtc27
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59269

libexec/rtld-elf/rtld.c

@@ -5462,6 +5462,11 @@ unref_dag(Obj_Entry *root)
 
 /*
  * Common code for MD __tls_get_addr().
+ *
+ * The tcb->tcb_dtv data structure is thread-local.  The reason that
+ * the function needs to take the rtld_bind_lock exclusive (as opposed
+ * to only shared, to safely access obj_list in allocate_module_tls())
+ * is to protect the rtld_malloc data.
  */
 static void *
 tls_get_addr_slow(struct tcb *tcb, int index, size_t offset, bool locked)