Mention the mattress.h header in the docstrings.

src/mattress/InitializedMatrix.py

@@ -22,14 +22,15 @@ def _factorize(group):
 
 
 class InitializedMatrix:
-    """Pointer to an initialized ``tatami::matrix``, for use in C++ code.
+    """Pointer to an object containing a ``tatami::Matrix``, typically
+    generated by :py:func:`~initialize.initialize` for use in C++ code.
     Instances of this class should only be created by developers and used
     within package functions; this is done by passing the :py:attr:`~ptr`
-    address to C++ and casting it to a ``mattress::BoundMatrix``. All
-    ``InitializedMatrix`` instances are expected to be transient within a
-    Python session; they should not be serialized, nor should they be visible
-    to end users. Each instance will automatically free the C++-allocated
-    memory upon garbage collection.
+    address to C++ and casting it to a pointer to a ``mattress::BoundMatrix``
+    (see the ``mattress.h`` header). All ``InitializedMatrix`` instances are
+    expected to be transient within a Python session; they should not be
+    serialized, nor should they be visible to end users. Each instance will
+    automatically free the C++-allocated memory upon garbage collection.
     """
 
     def __init__(self, ptr: int):
@@ -66,9 +67,9 @@ def shape(self) -> Tuple[int, int]:
         return lib.extract_dim(self._ptr)
 
     @property
-    def ptr(self): 
-        """An address to a ``mattress::BoundMatrix`` instance, to be passed
-        as a ``uintptr_t`` to C++ for casting."""
+    def ptr(self) -> int:
+        """Address to a ``mattress::BoundMatrix`` instance, to be passed as a
+        ``uintptr_t`` to C++ - see the ``mattress.h`` header."""
         return self._ptr
 
     @property