Bug Report

Describe the bug
improve configuration parameter descriptions

- shared_key: clarify it's for secure forward authentication
- self_hostname: explain it's used in handshake for secure forward auth
- unix_perm: add trailing period for consistency
- empty_shared_key: clarify it enables empty string as shared key

To Reproduce
Just inconsistent config parameter descriptions or very short one word outputs.

Expected behavior
Wanted to improve the config parameter output when using:

./fluent -i forward -h

Screenshots
N/A

Your Environment

• Version used: all
• Configuration: all
• Environment name and version (e.g. Kubernetes? What version?): all
• Server type and version: all
• Operating System and version: all
• Filters and plugins: in_forward

Additional context
Unclear and inconsistent help description for in_forward plugin.