[Identity] Document JSON.stringify() fallback for various password managers

[javiercn]

Some password managers do not implement PublicKeyCredential.prototype.toJSON correctly, which is required for JSON.stringify() to work when serializing a passkey credential - e.g.

See #62917

[MihuBot]

Possible related and/or duplicate issues:

• Passkeys - Illegal invocation with 1Password #62916
• Passkeys - JSON.stringify() fallback for various password managers #62917