[DOCS] Fix ignore_unavailable parameter definition (elastic#84071)

The current `ignore_unavailable` definition is a bit misleading. The parameter primarily determines if a request that targets a missing or closed index returns an error.

Author: jrodewig

docs/reference/eql/eql-search-api.asciidoc

@@ -109,8 +109,8 @@ Comma-separated list of filters for the API response. See
 <<common-options-response-filtering>>.
 
 `ignore_unavailable`::
-(Optional, Boolean) If `true`, missing or closed indices are not included in the
-response. Defaults to `true`.
+(Optional, Boolean) If `false`, the request returns an error if it targets a
+missing or closed index. Defaults to `true`.
 
 `keep_alive`::
 +

docs/reference/rest-api/common-parms.asciidoc

@@ -398,8 +398,8 @@ end::ignore_throttled[]
 
 tag::index-ignore-unavailable[]
 `ignore_unavailable`::
-(Optional, Boolean) If `true`, missing or closed indices are not included in the
-response. Defaults to `false`.
+(Optional, Boolean) If `false`, the request returns an error if it targets a
+missing or closed index. Defaults to `false`.
 end::index-ignore-unavailable[]
 
 tag::include-defaults[]