CONTRIBUTING.md: update init/finish description

CONTRIBUTING.md

@@ -167,7 +167,6 @@ if (condition1 && condition2 && ...
 
 Try to break the line in the place which you think is the most appropriate.
 
-
 ### Line Length
 
 Try to keep your lines under 80 columns, but you can go up to 100 if it
@@ -190,16 +189,18 @@ Functions that are responsible for constructing objects should take one of the
 two following forms:
 
 * `init`: for functions which accept a pointer to a pre-allocated object (e.g.
-a member of a struct) and initialize it.
+a member of a struct) and initialize it. Such functions must call `memset()`
+to zero out the memory before initializing it to avoid leaving unset fields.
 * `create`: for functions which allocate the memory for an object, initialize
-it, and return a pointer.
+it, and return a pointer. Such functions should allocate the memory with
+`calloc()` to avoid leaving unset fields.
 
 Likewise, functions that are responsible for destructing objects should take
 one of the two following forms:
 
 * `finish`: for functions which accept a pointer to an object and deinitialize
-it. Such functions should always be able to accept an already deinitialized
-object.
+it. If a finished object isn't destroyed but kept for future use, it must be
+reinitialized to be used again.
 * `destroy`: for functions which accept a pointer to an object, deinitialize
 it, and free the memory. Such functions should always be able to accept a NULL
 pointer.