Update docs/internal/3i.md

Co-authored-by: Justin Cappos <[email protected]>

Author: Yaxuan-w

docs/internal/3i.md

@@ -115,7 +115,7 @@ Represents a single Grate function entry shared between the lind-3i crate in was
 
 |   Field      |         Type       |                             FunDescriptionction                                   | 
 |--------------|--------------------|-----------------------------------------------------------------------------------|
-|   `fn_ptr`   |  `*const c_void`   | A C-ABI callback function pointer that re-enters the target Wasm module           |
+|   `fn_ptr`   |  `*const c_void`   | A C-ABI callback function pointer that re-enters the target Wasm module. This is used to jump back into the appropriate call-stack when the system call returns      |
 |   `ctx_ptr`  | `*const VMContext` | Pointer to the instance’s runtime context required by Wasmtime during re-entry    |
 
 `GrateFnEntry` instances are heap-allocated and referenced by 3i through stable raw pointers.