add comment to 'container' in 'quicklist.h'
add a comment to `container` in `quicklist.h`.
Because `PLAIN` and `PACKED` are not as easy to understand as `NONE`
and `LISTPACK` and we don't have a detailed comment on it.
* We use bit fields keep the quicklistNode at 32 bytes.
* count: 16 bits, max 65536 (max lp bytes is 65k, so max count actually < 32k).
* encoding: 2 bits, RAW=1, LZF=2.
- * container: 2 bits, PLAIN=1, PACKED=2.
+ * container: 2 bits, PLAIN=1 (a single item as char array), PACKED=2 (listpack with multiple items).
* recompress: 1 bit, bool, true if node is temporary decompressed for usage.
* attempted_compress: 1 bit, boolean, used for verifying during testing.
* extra: 10 bits, free for future use; pads out the remainder of 32 bits */
