docs: correct four statements checked against the client

- results: on a failed call getData() is null only when there is no JSON
  answer; when Proxmox VE answers with an error it is an empty node.
- tasks: same wording for the UPID of a failed call; the interval and the
  time of waitForTaskToFinish have a lower limit.
- API structure: required parameters are int and boolean, the optional ones
  Integer and Boolean.

Author: franklupo

docs/src/content/docs/concepts/api-structure.mdx

@@ -78,7 +78,8 @@ snapshot = vm.getSnapshot().snapshot("before-update", null, true);
 ```
 
 - Names are those of the API. A `-` in a name becomes `_`: `force-cpu` is `force_cpu`.
-- Booleans are `Boolean` and are sent as `1` or `0`.
+- Required parameters have primitive types (`int`, `boolean`), the optional ones the wrapper types
+  (`Integer`, `Boolean`), so that they can be `null`. Booleans are sent as `1` or `0`.
 - Parameters that exist with an index (`net0`, `net1`, `scsi0`) are one `Map<Integer, String>`
   argument, `netN`, `scsiN`: see [Indexed parameters](/cv4pve-api-java/concepts/indexed-parameters/).
 - Type, format and allowed values of each parameter are in the API viewer; the client sends what you

docs/src/content/docs/concepts/results.mdx

@@ -60,8 +60,9 @@ var upid = client.getNodes().get("pve01").getQemu().get(100).getStatus().getStar
 <Aside type="caution">
 `get("name")` returns `null` for a member that Proxmox VE leaves out, and many are optional: for
 example `name`, `cores` and `description` in the configuration of a VM, which are not returned when
-they have no value or the default one. On a failed call `getData()` itself is `null`. Check
-`isSuccessStatusCode()` first, and read optional members with `path`.
+they have no value or the default one. On a failed call there is no data: `getData()` is `null` when
+the request got no JSON answer, and an empty node, whose `get("name")` returns `null`, when Proxmox VE
+answered with an error. Check `isSuccessStatusCode()` first, and read optional members with `path`.
 </Aside>
 
 ### Optional members

docs/src/content/docs/concepts/tasks.mdx

@@ -37,11 +37,13 @@ var finished = client.waitForTaskToFinish(upid, 2000, 600000);
 
 It reads the status of the task at each interval until the task stops, and returns `true` if it
 stopped, `false` if it is still running when the time runs out. Set the time to the longest the task
-may take: seconds for a start or a snapshot, minutes for a clone, a migration or a backup.
+may take: seconds for a start or a snapshot, minutes for a clone, a migration or a backup. An interval
+of 0 or less becomes 500 milliseconds, and a time shorter than the interval becomes the interval plus
+5 seconds.
 
 <Aside type="caution">
 Check `isSuccessStatusCode()` of the call before reading the UPID, as above: a call that failed has
-no task, and `getData()` is `null`.
+no task, and no data to read the UPID from.
 </Aside>
 
 To check without waiting, `taskIsRunning(upid)` returns `true` while the task runs.