[skip changelog] Document the boards.txt vid and pid properties

[per1234]

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)

• What kind of change does this PR introduce?

Documentation update.

• What is the current behavior?

The boards.txt vid and pid properties are not documented. These properties are becoming increasingly important as the modern Arduino development software takes advantage of them for features based on automatic board detection (e.g., Arduino Web Editor's automatic board selection, arduino-cli board attach <port>)

• What is the new behavior?

The vid and pid properties of boards.txt are documented in the platform specification.

• Does this PR introduce a breaking change?

No.