[Refactor] Add comments for Metadata classes in attention module (#5789)

### What this PR does / why we need it?

Add docstrings for Metadata and MetadataBuilder classes in the attention
module to improve code readability.

Related to #5463 (Item 11: Add some comments for CommonMetadata and
others)

**Modified files:**
- `vllm_ascend/attention/context_parallel/common_cp.py`: Added comments
for `AscendPCPMetadata`, `CPChunkedContextMetadata`,
`AscendMetadataForPrefill`, `AscendMetadataForDecode`
- `vllm_ascend/attention/utils.py`: Added comments for
`AscendPrefillContextParallelMetadata`
- `vllm_ascend/attention/mla_v1.py`: Added comments for
`ChunkedContextMetadata`, `AscendMLADecodeMetadata`
- `vllm_ascend/attention/attention_v1.py`: Added comments for
`AscendMetadata`, `AscendAttentionMetadataBuilder`
- `vllm_ascend/attention/context_parallel/attention_cp.py`: Added
comments for `AscendAttentionCPMetadataBuilder`

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Documentation only, no functional changes.

Signed-off-by: lico67373 <918688502@qq.com>

vllm_ascend/attention/mla_v1.py

@@ -84,8 +84,11 @@ class AscendMLABackend(AttentionBackend):
 
 @dataclass
 class ChunkedContextMetadata:
-    # New for MLA (compared to FlashAttention)
-    # For handling chunked prefill
+    """
+    Metadata for chunked context handling in MLA attention.
+
+    Manages sequence boundaries and workspace for chunked prefill processing.
+    """
     cu_seq_lens: torch.Tensor
     starts: torch.Tensor
     seq_tot: list[int]
@@ -116,7 +119,8 @@ class AscendMLAPrefillMetadata:
 
 @dataclass
 class AscendMLADecodeMetadata:
-    # Input positions for rotrary embeddings since for MLA the rotary
+    """ Decode-specific metadata for Ascend MLA attention."""
+    # Input positions for rotary embeddings since for MLA the rotary
     # position embeddings are applied inside the attention backend
     input_positions: torch.Tensor
     block_table: torch.Tensor