dmctl: refine commands' usage text

[csuzhangxc]

What problem does this PR solve?

refine dmctl comands' usage text

What is changed and how it works?

refine usage suggestion from I18N team in pingcap/docs-cn#1878.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  1. run dmctl
  2. observe the output of usage

Related changes

• Need to cherry-pick to the release branch

[codecov]

Codecov Report

Merging #296 into master will increase coverage by 0.0135%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             master       #296        +/-   ##
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+ Coverage   60.2407%   60.2542%   +0.0135%     
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  Files           134        134                
  Lines         14789      14789                
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+ Hits           8909       8911         +2     
+ Misses         5043       5042         -1     
+ Partials        837        836         -1

[csuzhangxc]

@amyangfei @WangXiangUSTC PTAL

[WangXiangUSTC]

LGTM

[amyangfei]

will all output be changed to DM-worker, such as
https://github.com/pingcap/dm/blob/master/dm/ctl/master/break_ddl_lock.go#L54

[csuzhangxc]

will all output be changed to DM-worker, such as
https://github.com/pingcap/dm/blob/master/dm/ctl/master/break_ddl_lock.go#L54

sounds good. should we need to changed to DM-worker for error class?

[amyangfei]

will all output be changed to DM-worker, such as
https://github.com/pingcap/dm/blob/master/dm/ctl/master/break_ddl_lock.go#L54

sounds good. should we need to changed to DM-worker for error class?

I feel ok to change to DM-worker in error class, but it is still hard to change all dm-worker to DM-worker, such as variable name used in the config file. What about your opinion @GregoryIan @WangXiangUSTC

[csuzhangxc]

@amyangfei changed to DM-worker for dmctl's command direct output, but not change other places in da2793e.

other places including:

• error class
• log
• some output in DM-ansible

[amyangfei]

LGTM

BTW I prefer to keep dm-worker in error class name, log, which is more tidy.
Comparing
[code=40066:class=dm-worker:scope=internal:level=high] ExecuteDDL timeout, try use query-status to query whether the DDL is still blocking
[code=40066:class=DM-worker:scope=internal:level=high] ExecuteDDL timeout, try use query-status to query whether the DDL is still blocking