The code was refactored in commit 062ae6f to use 'protocol' instead of 'alpn'
for the ALPN protocol identifier option. However, the documentation was not updated
to reflect this change.

This commit updates the documentation to match the actual implementation:

• Removes the non-functional 'alpn' option from SessionOptions
• Adds the 'protocol' option with proper description
• Updates example code to use 'protocol' instead of 'alpn'
• Updates OnHandshakeCallback parameter from 'alpn' to 'protocol'

Why this change is necessary:

The destructuring assignment in processTlsOptions() expects a protocol property:
pt
const { protocol } = tls;

Therefore, using alpn will not work:

// This won't work
const alpn = 'h3';
connect(address, { alpn }); // protocol will be undefined

// This is correct
const protocol = 'h3';
connect(address, { protocol });

Refs: 062ae6f3cb5