Add max-days and max-backups default value 0 means for PD (#9330)

[ti-chi-bot]

This is an automated cherry-pick of #9330

What is changed, added or deleted? (Required)

This pull request adds max-days and max-backups default value 0 means for PD configuration guide. There is a similar document for TiKV's max-days and max-backups via #7425 There is no PD one.

Here are the technical backgrounds for these documentation changes.

1. max-days at PD configuration

https://github.com/tikv/pd/blob/b5e113436c85f81d0e6ca2b5b592271114089db2/conf/config.toml#L97

# max-days = 0

https://github.com/tikv/pd/blob/b5e113436c85f81d0e6ca2b5b592271114089db2/pkg/logutil/log.go#L33-L34

// Max log keep days, default is never deleting.
  MaxDays int `toml:"max-days" json:"max-days"`

It says "default is never deleting."

2. max-backups at PD configuration

https://github.com/tikv/pd/blob/b5e113436c85f81d0e6ca2b5b592271114089db2/conf/config.toml#L99

# max-backups = 0

https://github.com/tikv/pd/blob/b5e113436c85f81d0e6ca2b5b592271114089db2/pkg/logutil/log.go#L35-L36

  // Maximum number of old log files to retain.
  MaxBackups int `toml:"max-backups" json:"max-backups"`

log.go does not anything about the default value. Based on the method lookup, I believe MaxBackups default behavior is defined by https://github.com/natefinch/lumberjack default value as follows.

https://github.com/pingcap/log/blob/07caf4f152b9d53fedb241dc5e5aaa282fdaa74b/log.go#L126

    MaxBackups: cfg.MaxBackups,

https://github.com/natefinch/lumberjack/blob/47ffae23317c5951a2a6267a069cf676edf53eb6/lumberjack.go#L96-L99

  // MaxBackups is the maximum number of old log files to retain.  The default
  // is to retain all old log files (though MaxAge may still cause them to get
  // deleted.)
  MaxBackups int `json:"maxbackups" yaml:"maxbackups"`

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Do your changes match any of the following descriptions?

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[Oreoxmt]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: 47c76d9