Document how declarative language feature contributions from different sources are merged

[Lemmingh]

The documentation currently shows that extensions can define basic editing support (declarative language feature) in various aspects.

However, when multiple extensions with overlapping contributions are enabled at the same time, what will happen?

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This is a real world headache, confusing both extension authors and end users.

For example, vscode.yaml and redhat.vscode-yaml both define

• Language definition and configuration (languages and API)

• TM grammar (grammars)

• Default editor configuration (configurationDefaults)

for language yaml.

Please clearly document how to deduce the outcome.