Configuring brokerClientAuthenticationParameter for AuthenticationToken plugin does not work as documented

**Describe the bug**
The documentation suggest that the proper way to reference a file with `brokerClientAuthenticationParameter` for JWT plugin is `brokerClientAuthenticationParameters={"file":"///path/to/proxy-token.txt"}`, which does not work. When you [look at the source](https://github.com/apache/pulsar/blob/v2.9.1/pulsar-client/src/main/java/org/apache/pulsar/client/impl/auth/AuthenticationToken.java#L79) you'll see that it parses a string like `file:/some/path` or `token:someToken`, otherwise it tried JSON which explicitly only looks for the `token` property. Runtime behavior confirms this, I get a long long stack trace pointing back to trying to look at the `token` property of the provided JSON. So either [the docs](https://pulsar.apache.org/docs/en/2.8.2/security-jwt/#enable-token-authentication-on-brokers) are wrong or the code is wrong.

**To Reproduce**
Steps to reproduce the behavior:
1. Setup [Client authentication using tokens based on JSON Web Tokens]
2. Instead of embeddeding the token directly into the config uncomment and use the alternative to load from a file below, which reads: `brokerClientAuthenticationParameters={"file":"///path/to/proxy-token.txt"}`

**Expected behavior**
The provided JSON parameter works as stated in the documentation, OR the documentation instead recommends the non-JSON parameters.

**Screenshots**
N/A

**Desktop (please complete the following information):**
- Ubuntu 20.04, but this is running in the official container.

**Additional context**
Add any other context about the problem here.

Start with AuthenticationToken.java at the linked source location and the security-jwt documentation section; compare the accepted brokerClientAuthenticationParameters formats with the documented JSON file form. Reproduce the configuration from the listed steps, then align the implementation or documentation so the expected token-file configuration is correct and verify token authentication.

Written by the indexing model from the issue text.