Fixed '--libraries' and '--library' flags description in compile command

[cmaglie]

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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?
Fixed '--libraries' and '--library' flags description in compile command

What is the current behavior?
the description obtained from arduino-cli compile -h is:

      --libraries strings            List of custom libraries dir paths separated by commas. Or can be used multiple times for multiple libraries dir paths.
      --library strings              List of paths to libraries root folders. Libraries set this way have top priority in case of conflicts. Can be used multiple times for different libraries.

and is wrong (well, at least is not very clear...)

• What is the new behavior?

      --libraries strings            Path to a collection of libraries. Contained libraries override the ones found in the default libraries folder. Can be used multiple times or entries can be comma separated.
      --library strings              Path to a single library’s root folder. Overrides other occurrences of the library found in default libraries folder or passed in via --libraries. Can be used multiple times or entries can be comma separated.

Does this PR introduce a breaking change, and is titled accordingly?
No

[per1234]

I'm wondering whether it is wise to attempt an explanation of library priority here. Location is only one of the factors.

[cmaglie]

I'd rather keep the usage help short...

BTW your suggestion made me wonder if we could improve the verbose compilation output by indicating, in addition to which library it was selected, also why it was selected.

[per1234]

I may not have communicated clearly in my previous review. I was not suggesting that we should add to it. I was suggesting that we remove the incomplete information about library priority.

[cmaglie]

That makes sense. I've also changed the description of the gRPC API.