PDD17: Mention init_int VTABLE

Message mostly copied from init_pmc explanation.  I figured a
repetitive description is better than none.

docs/pdds/pdd17_pmc.pod

@@ -434,6 +434,16 @@ null value passed as the initializer parameter is allowed.
 NOTE: It is strongly suggested that init_pmc(PMCNULL) be equivalent to
 init(), though there will of necessity be exceptions.
 
+=item init_init
+
+  void init_int(INTERP, PMC *self, INTVAL initializer)
+
+Alternative entry point called when a PMC is first instantiated.  Accepts an
+integer parameter used to initialize the given object.  Interpretation of the
+integer initializer is left open, each PMC is free to choose its own
+implementation.  This is very useful for PMCs that require an integer size
+parameter and is used as such in FixedPMCArray and related.
+
 =item instantiate
 
   PMC* instantiate(INTERP, PMC *self, PMC *init)