Improve based on review comments

Author: jepler

shared-bindings/jpegio/JpegDecoder.c

@@ -33,7 +33,11 @@
 #include "shared-module/displayio/Bitmap.h"
 
 //| class JpegDecoder:
-//|     """A JPEG decoder"""
+//|     """A JPEG decoder
+//|
+//|     A JPEG decoder allocates a few thousand bytes of memory. To reduce memory fragmentation,
+//|     create a single JpegDecoder object and use it anytime a JPEG image needs to be decoded.
+//|     """
 //|
 //|     def __init__(self) -> None: ...
 //|
@@ -45,6 +49,7 @@ STATIC mp_obj_t jpegio_jpegdecoder_make_new(const mp_obj_type_t *type, size_t n_
 
     jpegio_jpegdecoder_obj_t *self = mp_obj_malloc(jpegio_jpegdecoder_obj_t, &jpegio_jpegdecoder_type);
     self->base.type = &jpegio_jpegdecoder_type;
+    common_hal_jpegio_jpegdecoder_construct(self);
 
     return MP_OBJ_FROM_PTR(self);
 }

shared-module/jpegio/JpegDecoder.c

@@ -30,6 +30,11 @@
 #include "shared-bindings/bitmaptools/__init__.h"
 #include "shared-module/jpegio/JpegDecoder.h"
 
+// Given a pointer `ptr` to the field `field_name` inside a structure of type `type`,
+// retrieve a pointer to the containing object.
+// This is used to retrieve the jpegio_jpegdecoder_obj_t object given the JDEC.
+// Similar macros of this type are frequently employed in low-level code, but this is
+// not standardized.
 #define CONTAINER_OF(ptr, type, field_name) ((type *)(void *)(((uint8_t *)ptr) - offsetof(type, field_name)))
 
 static void check_jresult(JRESULT j) {