Add high-level comment about index pools

Include/internal/pycore_index_pool.h

@@ -13,6 +13,11 @@ extern "C" {
 
 #ifdef Py_GIL_DISABLED
 
+// This contains code for allocating unique indices in an array. It is used by
+// the free-threaded build to assign each thread a globally unique index into
+// each code object's thread-local bytecode array.
+
+// A min-heap of indices
 typedef struct _PyIndexHeap {
     Py_ssize_t *values;
 
@@ -23,6 +28,8 @@ typedef struct _PyIndexHeap {
     Py_ssize_t capacity;
 } _PyIndexHeap;
 
+// An unbounded pool of indices. Indices are allocated starting from 0. They
+// may be released back to the pool once they are no longer in use.
 typedef struct _PyIndexPool {
     PyMutex mutex;