改成这样，根节点给注释了。
# WARNING: This file is auto-generated. Do not edit! All your modification will be overwritten!
# You can use 'tiup cluster edit-config' and 'tiup cluster reload' to update the configuration
# All configuration items you want to change can be added to:
server_configs:
tidb:
# aa.b1.c3: value
# aa.b2.c4: value
mem-quota-query = 4294967296
[log]
slow-threshold = "300i"
[performance]
txn-total-size-limit = 1073741824