Add comments to the initial-store-count attribute in pd application.yml

[haohao0103]

Bug Type (问题类型)

None

Before submit

• 我已经确认现有的 Issues 与 FAQ 中没有相同 / 重复问题 (I have confirmed and searched that there are no similar problems in the historical issue and documents)

Environment (环境信息)

• Server Version: 1.5.0 (Apache Release Version)
• Backend: RocksDB 5 nodes, SSD

Expected & Actual behavior (期望与实际表现)

PD cluster will assess the overall health of the cluster. If the number of store nodes is less than the initial-store-count, the cluster will be unavailable

Vertex/Edge example (问题点 / 边数据举例)

No response

Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)

No response