add detailed docs to ref_from_ptr

src/instance.rs

@@ -139,6 +139,16 @@ impl<'py> Bound<'py, PyAny> {
         Py::from_owned_ptr_or_err(py, ptr).map(|obj| Self(py, ManuallyDrop::new(obj)))
     }
 
+    /// This slightly strange method is used to obtain `&Bound<PyAny>` from a pointer in macro code
+    /// where we need to constrain the lifetime `'a` safely.
+    ///
+    /// Note that `'py` is required to outlive `'a` implicitly by the nature of the fact that
+    /// `&'a Bound<'py>` means that `Bound<'py>` exists for at least the lifetime `'a`.
+    ///
+    /// # Safety
+    /// - `ptr` must be a valid pointer to a Python object for the lifetime `'a`. The `ptr` can
+    ///   be either a borrowed reference or an owned reference, it does not matter, as this is
+    ///   just `&Bound` there will never be any ownership transfer.
     #[inline]
     pub(crate) unsafe fn ref_from_ptr<'a>(
         _py: Python<'py>,