[skip-changelog] Modify board search --help description to correctly explain the command's function

[MatteoPologruto]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Documentation imperfection fix

What is the current behavior?

board search and board listall have the same --help description. The first one is wrong.

PS C:\Users\m.pologruto\Desktop\Arduino\arduino-cli> arduino-cli board --help

Available Commands:
  attach      Attaches a sketch to a board.
  details     Print details about a board.
  list        List connected boards.
  listall     List all known boards and their corresponding FQBN.
  search      List all known boards and their corresponding FQBN.

What is the new behavior?

board search --help description has been changed to correctly reflect the command's function.

PS C:\Users\m.pologruto\Desktop\Arduino\arduino-cli> arduino-cli board --help

Available Commands:
  attach      Attaches a sketch to a board.
  details     Print details about a board.
  list        List connected boards.
  listall     List all known boards and their corresponding FQBN.
  search      Search for a board in Boards Manager.

Does this PR introduce a breaking change, and is titled accordingly?

No

[codecov]

Codecov Report

Base: 36.62% // Head: 36.56% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (ee9ae9d) compared to base (940c945).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2081      +/-   ##
==========================================
- Coverage   36.62%   36.56%   -0.06%     
==========================================
  Files         229      229              
  Lines       19539    19538       -1     
==========================================
- Hits         7156     7144      -12     
- Misses      11547    11555       +8     
- Partials      836      839       +3     

Flag	Coverage Δ
unit	36.56% <0.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/cli/board/search.go	0.00% <0.00%> (ø)
arduino/monitor/monitor.go	41.05% <0.00%> (-6.32%)	⬇️

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