[Docs] Setting docs for different modes are confusing

[robbavey]

The text around settings in the mode section of the documentation is somewhat ambiguous and can be misconstrued:

If `read` is specified, these settings can be used:

* `ignore_older` (older files are not processed)
* `file_completed_action` (what action should be taken when the file is processed)
* `file_completed_log_path` (which file should the completed file path be logged to)

If `read` is specified, these settings are ignored:

* `start_position` (files are always read from the beginning)
* `close_older` (files are automatically 'closed' when EOF is reached)

For example, ignore_older is applicable to both tail and read, file_completed... are both applicable to read only. It may make sense to rework this section to improve clarity.

[TheVastyDeep]

I think the documentation should explicitly say that for a file input setting ignore_older to zero causes any file more than zero seconds old (likely all of them) to be ignored. It is different to filebeat, which setting it to zero disables the function.