varchar value should use quotation marks

Error Report

Please answer the following questions before submitting your issue. Thanks!

1. What is the URL/path of the document related to this issue?

https://docs.pingcap.com/tidb/stable/garbage-collection-configuration
tikv_gc_enable
Enables or disables GC
Default: true

2. How would you like to improve it?
   tikv_gc_enable
   Enables or disables GC
   Default:" true"

all variable value's type is varchar, so it should be include in quotes.

desc mysql.tidb;
+----------------+---------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+---------------+------+------+---------+-------+
| VARIABLE_NAME | varchar(64) | NO | PRI | NULL | |
| VARIABLE_VALUE | varchar(1024) | YES | | NULL | |
| COMMENT | varchar(1024) | YES | | NULL | |
+----------------+---------------+------+------+---------+-------+