output/exec_filter: document deprecated parameters and section defaults

[Watson1978]

Five parameters are declared with deprecated: in out_exec_filter.rb but none of them appeared in the documentation. Users had no way to learn what they do or what to use instead.

• remove_prefix (0.10.9) use @label instead
• add_prefix (0.10.9) use @label instead
• tag_key (0.14.9) use tag_key in <inject>/<extract>
• time_key (0.10.5) use time_key in <inject>/<extract>
• time_format (0.10.5) use time_format in <inject>/<extract>

All five became deprecated in v0.14.9 (fcc4144f), which the text states. The version column is the release which first accepted the parameter, confirmed against the release tag contents rather than guessed.

tag_key is the odd one, and the text explains it: v0.14.9 both added and deprecated it. It was introduced in 0.10.5, dropped during the v0.12 series (eb3ff406 replaced it with in_tag_key / out_tag_key), and fcc4144f reinstated it as a compatibility alias with deprecated: already attached. v0.14.8 has no declaration at all, so 0.14.9 is correct for the parameter as it exists today even though it looks like a typo next to time_key at 0.10.5.

Three related fixes on the same page:

• ## Parameters had no link to Common Parameters, unlike every other output plugin page.
• The <inject> section's time_type default is unixtime, not float. The plugin overrides the inject helper default with config_set_default :time_type, :unixtime.
• The <parse> section's estimate_current_event was missing. The plugin overrides the parser default true with false.

Both time_type entries now note that specifying time_format in the same section makes time_type string, which is easy to miss because it happens in configure before the section is parsed.