Document multiple Flutter plugin instances

[awnoa]

Description of what this PR is changing or adding, and why:

Documents that an app can run multiple Flutter engines, that each engine creates its own plugin instance, and that those instances can have independent lifetimes. It also adds guidance for scoping engine-specific state and coordinating shared native resources.

Issues fixed by this PR (if any):

Fixes flutter/flutter#150855.

PRs or commits this PR depends on (if any):

None.

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[gemini-code-assist]

Code Review

This pull request adds a new section to the package development documentation detailing how to support multiple Flutter instances. It advises developers to avoid global or static variables for engine-specific state and to coordinate access to shared native singletons. The reviewer suggested adding a concrete example of coordination, such as reference counting, to make the guidance more actionable.