[skip changelog] Make the command documentation of cache clean more clear

[per1234]

The terms "cache" and "clean" might bring compilation to mind, but the arduino-cli cache clean command has nothing to
do with compilation. Many users will likely not even be aware of the existence or purpose of the directories.downloads
folder. For this reason, it's important to make the purpose of this command clear.

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

docs update

• What is the current behavior?

The terms "cache" and "clean" might bring compilation to mind, but the arduino-cli cache clean command has nothing to
do with compilation. Many users will likely not even be aware of the existence or purpose of the directories.downloads
folder. It's not very clear from the command documentation for arduino-cli cache clean what it does.

For example:
#969 (comment)

At first glance arduino-cli cache clean might do this, but the --help output does not specify what it cleans exactly, cache clean -v does not show what is removed either, and a quick test suggests that it does not clean the build directory.

• What is the new behavior?

The purpose of the arduino-cli cache clean is more clearly described in the command line documentation.

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

No

• Other information:

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