[Doc] Remove outdated comment to avoid misunderstanding

The comment previously mentioned that prefix caching and sliding-window
were incomplete, but these features have now been implemented [Issue
4537](#4537)

vllm/core/block_manager_v2.py

@@ -24,9 +24,8 @@ class BlockSpaceManagerV2(BlockSpaceManager):
     autoregressively-generated tokens, and other advanced features such as
     prefix caching, forking/copy-on-write, and sliding-window memory allocation.
 
-    The current implementation is partial; in particular prefix caching and
-    sliding-window are not feature complete. This class implements the design
-    described in https://github.com/vllm-project/vllm/pull/3492.
+    This class implements the design described in
+    https://github.com/vllm-project/vllm/pull/3492.
 
     Lookahead slots
         The block manager has the notion of a "lookahead slot". These are slots
@@ -190,7 +189,7 @@ def allocate(self, seq_group: SequenceGroup) -> None:
 
         assert (request_id
                 not in self.cross_block_tables), \
-                "block table already exists"
+            "block table already exists"
 
         check_no_caching_or_swa_for_blockmgr_encdec(self, seq_group)