CONTRIBUTING.md: update construction/destruction functions' description

wlroots doesn't really follow the rule of keeping `create`/`destroy` and
`init`/`finish` functions in pairs, so the relevant doc section is
updated accordingly.

CONTRIBUTING.md

@@ -186,16 +186,23 @@ all of the characters, and replace any invalid characters with an underscore.
 
 ### Construction/Destruction Functions
 
-For functions that are responsible for constructing and destructing an object,
+Functions that are responsible for constructing objects should take one of the
-they should be written as a pair of one of two forms:
+two following forms:
-* `init`/`finish`: These initialize/deinitialize a type, but are **NOT**
+
-responsible for allocating it. They should accept a pointer to some
+* `init`: for functions which accept a pointer to a pre-allocated object (e.g.
-pre-allocated memory (e.g. a member of a struct).
+a member of a struct) and initialize it.
-* `create`/`destroy`: These also initialize/deinitialize, but will return a
+* `create`: for functions which allocate the memory for an object, initialize
-pointer to a `malloc`ed chunk of memory, and will `free` it in `destroy`.
+it, and return a pointer.
-
+
-A destruction function should always be able to accept a NULL pointer or a
+Likewise, functions that are responsible for destructing objects should take
-zeroed value and exit cleanly; this simplifies error handling a lot.
+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.
+* `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.
 
 ### Error Codes