[skip changelog] Document compiler.libraries.ldflags property in platform specification

[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?

The Arduino platform specification provides no documentation for the precompiled libraries system.

• What is the new behavior?

Documentation on what is required for platforms to support precompiled libraries is provided by the platform specification.

• Does this PR introduce a breaking change?

No.

• Other information:

I'm unsure on what the best approach is for documenting which recipes can use {compiler.libraries.ldflags} (and properties in general). This property is only used by official platforms in recipe.c.combine.pattern, so I documented it in that section, but that gives the impression that it's only accessible in that recipe, which is not true. I don't know whether there is an application for it elsewhere, but anything is possible in the world of 3rd party platforms