MS-574 update config comments

Former-commit-id: d9b088a15e16c43be8eae1657184d30ade8e3053

cpp/conf/server_config.template

-# 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)
@@ -25,8 +25,8 @@ metric_config:
 
 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