MS-574 update config comments

# All the following configurations are default values.

# Default values are used when you make no changes to the following parameters.

server_config:

  address: 0.0.0.0                  # milvus server ip address (IPv4)

cache_config:

  cpu_mem_capacity: 16              # GB, CPU memory used for cache

  cpu_mem_threshold: 0.85           # percentage of data kept when cache cleanup triggered

  cache_insert_data: false          # whether load inserted data into cache

  cpu_mem_threshold: 0.85           # percentage of data that will be kept when cache cleanup is triggered

  cache_insert_data: false          # whether to load inserted data into cache

engine_config:

  blas_threshold: 20