manual: Add missing section and node for clockid_t wait functions

Without this, these functions appear under the node Default Thread
Attributes, which is confusing.

Eventually, the documentation should be merged with the (yet to be
documented) standardized functions.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

manual/threads.texi

@@ -625,6 +625,8 @@ the standard.
 @menu
 * Default Thread Attributes::             Setting default attributes for
 					  threads in a process.
+* Waiting with Explicit Clocks::          Functions for waiting with an
+                                          explicit clock specification.
 @end menu
 
 @node Default Thread Attributes
@@ -669,6 +671,12 @@ The system does not have sufficient memory.
 @end table
 @end deftypefun
 
+@node Waiting with Explicit Clocks
+@subsubsection Functions for Waiting According to a Specific Clock
+
+@Theglibc{} provides several waiting functions that expect an explicit
+@code{clockid_t} argument.
+
 @comment semaphore.h
 @comment POSIX-proposed
 @deftypefun int sem_clockwait (sem_t *@var{sem}, clockid_t @var{clockid},