Clarify list_add documentation

Originally, the function list_add is said to insert a node at the
beginning of a given list. However, it could also be used to insert a
node after any existing node in the list. Therefore, this commit
modifies its documentation to reflect the actual list manipulation
performed by the function.

Change-Id: I661d7a0ebb876712c8ff75deef84370a51e62f1c

Author: SimonLiu423

scripts/checksums

@@ -1,3 +1,3 @@
-bff0704601c8197ae6b847647b4167664b7d6abc  queue.h
-b26e079496803ebe318174bda5850d2cce1fd0c1  list.h
-1029c2784b4cae3909190c64f53a06cba12ea38e  scripts/check-commitlog.sh
+398b59c23f699ff1bf1e73a94503a9a91caa9207  queue.h
+5179cf8f5aafbc6cb8e8d7b898ef3798cfe85613  list.h
+1029c2784b4cae3909190c64f53a06cba12ea38e  scripts/check-commitlog.sh