Make space for documentation aside during followup completion select (#21716)

The goal of #7115 appears to be to limit the disruptiveness of
completion documentation load causing the completion selector to move
around. The approach was to debounce load of documentation via a setting
`completion_documentation_secondary_query_debounce`. This particularly
had a nonideal interaction with #21286, where now this debounce interval
was used between the documentation fetches of every individual
completion item.

I think a better solution is to continue making space for documentation
to be shown as soon as any documentation is shown. #21704 implemented
part of this, but it did not persist across followup completions.

Release Notes:

- Fixed completion list moving around on load of documentation. The
previous approach to mitigating this was to rate-limit the fetch of
docs, configured by a
`completion_documentation_secondary_query_debounce` setting, which is
now deprecated.

docs/src/configuring-zed.md

@@ -1517,16 +1517,6 @@ Or to set a `socks5` proxy:
 
 `boolean` values
 
-## Completion Documentation Debounce Delay
-
-- Description: The debounce delay before re-querying the language server for completion documentation when not included in original completion list.
-- Setting: `completion_documentation_secondary_query_debounce`
-- Default: `300` ms
-
-**Options**
-
-`integer` values
-
 ## Show Inline Completions
 
 - Description: Whether to show inline completions as you type or manually by triggering `editor::ShowInlineCompletion`.