docs(firebase_auth): add note that persistence is only available on web based platforms.

[nilsreichardt]

Description

Copied from:

flutterfire/packages/firebase_auth/firebase_auth_platform_interface/lib/src/types.dart

Line 32 in c2771a4

/// Setting a persistence type is only available on web based platforms.

Related Issues

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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[nilsreichardt]

4 PR approvals for 1 one line doc comment 😂