[skip changelog] Document build.library_discovery_phase property in platform specification

[per1234]

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• 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?

Docs update.

• What is the current behavior?

The build.library_discovery_phase property is undocumented.

• What is the new behavior?

The build.library_discovery_phase property is documented in the Arduino platform specification.

• Does this PR introduce a breaking change?

No.

• Other information:

Introduction of ARDUINO_LIB_DISCOVERY_PHASE macro: #633
Introduction of build.library_discovery_phase property: #838

Note: I am guessing that the next release versions will be:

• Arduino CLI: 0.12.0
• Arduino Builder: 1.6.0
• Arduino IDE; 1.8.14

If things go differently, the version numbers specified in this documentation will need to be corrected.