[Doc] improvements and clarifications

[karenzone]

This PR:

• fills in datatype for proxy
• adds a header to separate two sets of options
• cleans up formatting for validate_after_inactivity

To Do:

• When resolved, make updates to input-http_poller.

[karenzone]

The formatting for validate_after_inactivity wasn't rendering correctly.

I took a guess at what you were going for, and reformatted and reworded it a bit.

Here's how it's rendering now:

[jsvd]

@karenzone well spotted, this was essentially a copy of the settings that all plugins that use the logstash-mixin-http_client has, since we don't have a nice way to fetch these from another repository.

So I copied over the settings from a plugin that uses this mixin - the http_poller input - which has the same issues

[karenzone]

Cool. When we get this right, I'll fix for input-http_poller

[jsvd]

@karenzone do you mind also bumping the version and changelog?