doc: more detailed --help

[YikSanChan]

Learn from spf13/cobra#571 (comment).

Note the difference in "Usage".

before:

$ featctl describe entity --help
show details of a specific entity

Usage:
  featctl describe entity [flags]

Flags:
  -h, --help   help for entity

Global Flags:
      --config string   config file (default "/Users/chenyisheng/.config/featctl/config.yaml")

after:

$ featctl describe entity --help
show details of a specific entity

Usage:
  featctl describe entity <entity_name> [flags]

Flags:
  -h, --help   help for entity

Global Flags:
      --config string   config file (default "/Users/chenyisheng/.config/featctl/config.yaml")

close #530

[codecov]

Codecov Report

Merging #531 (8271909) into main (6e1cf67) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #531   +/-   ##
=======================================
  Coverage   57.38%   57.38%           
=======================================
  Files          38       38           
  Lines        1605     1605           
=======================================
  Hits          921      921           
  Misses        552      552           
  Partials      132      132           

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[wfxr]

LGTM