[Docs] yugabyted ysql_hba_conf_csv/ysql_pg_conf_csv curly brace syntax

[jasonyb]

Description

yugabyted's ysql_hba_conf_csv and ysql_pg_conf_csv flag syntax changed in commit 96d6e2b to take curly braces. The syntax is not documented in the script nor in the docs. So I turned to old commits messages for a reference and found commits like 197d4c9 and d9b0de9 using the old syntax, such as

bin/yugabyted start --tserver_flags "vmodule=intent_aware_iterator=4,pg_savepoints_enabled=true,yb_savepoints_enabled=true"

and trying to copy that, got unhelpful error

Starting yugabyted...
yugabyted crashed. For troubleshooting, contact us on https://www.yugabyte.com/slack or check our FAQ at https://docs.yugabyte.com/latest/faq/
Traceback (most recent call last):
  File "./bin/yugabyted", line 4792, in run
    args.func()
  File "./bin/yugabyted", line 419, in start
    self.start_processes()
  File "./bin/yugabyted", line 1536, in start_processes
    yb_tserver_cmd = self.get_tserver_cmd(common_gflags)
  File "./bin/yugabyted", line 2148, in get_tserver_cmd
    if ysql_pg_conf_csv_flag[0] == "'" and ysql_pg_conf_csv_flag[-1] == "'":
IndexError: string index out of range

A different issue can be opened for making the error message better, but at least document this somewhere such as reference/configuration/yugabyted.md. It's very unintuitive to put in curly braces.

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[jasonyb]

I further realize that the change was only made to those specific flags but not others like vmodule, which logically should behave the same way. It seems to me the approach of only handling curly brace syntax for specific hand-picked flags is not ideal since those kinds of csv flags will come and go all the time, so you need to modify yugabyted every time.