Clarify wait_for_completion doc in the Task API (#93754)

docs/reference/cluster/tasks.asciidoc

@@ -51,7 +51,7 @@ include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=parent-task-id]
 include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=timeoutparms]
 
 `wait_for_completion`::
-(Optional, Boolean) If `true`, the request blocks until the operation is complete.
+(Optional, Boolean) If `true`, the request blocks until all found tasks are complete.
 Defaults to `false`.
 
 [[tasks-api-response-codes]]