[skip changelog] Document library.properties precompiled field's "full" option

[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?

Docs update.

• What is the current behavior?

Previously, the precompiled field of library.properties only supported one value: true. As it became apparent that there were two distinct use cases for this feature (arduino/arduino-builder#353), it was necessary to add a new full option so that both use cases could be fully supported (#611). However, the full option is not documented.

• What is the new behavior?

The precompiled field of library.properties' full value is documented in the appropriate section of the Arduino library specification.

• Does this PR introduce a breaking change?

No.

• Other information:

The undocumented nature of this option was noted in #754