[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/context_parallel/attention_cp.py

@@ -45,7 +45,11 @@ from vllm_ascend.utils import cp_chunkedprefill_comm_stream, weak_ref_tensors
 
 
 class AscendAttentionCPMetadataBuilder(AscendAttentionMetadataBuilder):
-    # AttentionCGSupport.UNIFORM_SINGLE_TOKEN_DECODE
+    """
+    Builder for constructing AscendMetadata with Context Parallelism support.
+
+    Extends AscendAttentionMetadataBuilder with PCP/DCP metadata handling.
+    """
     # Does this backend/builder reorder the batch?
     # If not, set this to None. Otherwise set it to the query
     # length that will be pulled into the front of the batch.