[clang] [clang][docs] Improve documentation of [[ownership_returns]] attribute (PR #191005)

[Balázs Benics via cfe-commits]

================
@@ -174,12 +174,21 @@ If a project uses custom functions for dynamic memory management (that e.g. act
 Attribute 'ownership_returns' (Clang-specific)
 ----------------------------------------------
 
-Use this attribute to mark functions that return dynamically allocated memory. Takes a single argument, the type of the allocation (e.g. ``malloc`` or ``new``).
+Use this attribute to mark functions that return dynamically allocated memory.
+The first argument is the type of the allocation (e.g. ``malloc``, ``new``, or any other identifier).
+An optional second argument is the 1-based index of the function parameter that specifies the allocation size in bytes.
+The referenced parameter must have an integral type.
+
+This attribute may appear at most once per function declaration.
 
 .. code-block:: c
 
+  // Without size parameter:
   void __attribute((ownership_returns(malloc))) *my_malloc(size_t);
----------------
steakhal wrote:

```suggestion
  void __attribute((ownership_returns(malloc))) *my_malloc(size_t sz);
```

https://github.com/llvm/llvm-project/pull/191005