chore(container): comments update

[gqcn]

This pull request updates the documentation for the isNil method across several container types to clarify that, when no custom nil checker is set, the method falls back to the default empty.IsNil function instead of using a standard nil check with any(v) == nil. No functional code changes are made—only comments are updated for accuracy and clarity.

Documentation consistency improvements:

• Updated the isNil method comments in KVMap, ListKVMap, and TSet to specify that the fallback is to empty.IsNil, not any(v) == nil. (container/gmap/gmap_hash_k_v_map.go, container/gmap/gmap_list_k_v_map.go, container/gset/gset_t_set.go) [1] [2] [3]
• Updated the isNil method comments in AVLKVTree, BKVTree, and RedBlackKVTree to match the new description regarding the fallback behavior. (container/gtree/gtree_k_v_avltree.go, container/gtree/gtree_k_v_btree.go, container/gtree/gtree_k_v_redblacktree.go) [1] [2] [3]