Add code comments for StgInfoTable and StgStack structs

includes/rts/storage/Closures.h

@@ -63,6 +63,11 @@ typedef struct {
    -------------------------------------------------------------------------- */
 
 typedef struct {
+    // If TABLES_NEXT_TO_CODE is defined, then `info` is offset by
+    // `sizeof(StgInfoTable)` and so points to the `code` field of the
+    // StgInfoTable! You may want to use `get_itbl` to get the pointer to the
+    // start of the info table. See
+    // https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/rts/storage/heap-objects#tables_next_to_code.
     const StgInfoTable* info;
 #if defined(PROFILING)
     StgProfHeader         prof;

includes/rts/storage/TSO.h

@@ -242,10 +242,22 @@ typedef struct StgTSO_ {
 
 typedef struct StgStack_ {
     StgHeader  header;
-    StgWord32  stack_size;     // stack size in *words*
+
+    /* Size of the `stack` field in *words*. This is not affected by how much of
+     * the stack space is used, nor if more stack space is linked to by an
+     * UNDERFLOW_FRAME.
+     */
+    StgWord32  stack_size;
+
     StgWord8   dirty;          // non-zero => dirty
     StgWord8   marking;        // non-zero => someone is currently marking the stack
-    StgPtr     sp;             // current stack pointer
+
+    /* Pointer to the "top" of the stack i.e. the most recently written address.
+     * The stack is filled downwards, so the "top" of the stack starts with `sp
+     * = stack + stack_size` and is decremented as the stack fills with data.
+     * See comment on "Invariants" below.
+     */
+    StgPtr     sp;
     StgWord    stack[];
 } StgStack;