[2.0.4] dmctl: support read master addr from env (#691)

[ti-chi-bot]

This is an automated cherry-pick of #691

What is changed, added, or deleted? (Required)

update dmctl doc to note that master_addr can be read from env var.

Which DM version(s) do your changes apply to? (Required)

• master (the latest development version, including v3.0 changes)
• v2.0 (TiDB DM 2.0 versions)

What is the related PR or file link(s)?

• This PR is translated from: dmctl: try read master addr from env (#1722) dm#1726

Do your changes match any of the following descriptions?

• Change aliases

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• CharLotteiu

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[CharLotteiu]

/merge

[ti-chi-bot]

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

Commit hash: da49a18