docs: correct four statements checked against the client

[franklupo]

Summary

A check of the whole documentation against the client found four statements that were not exact. No code change.

• concepts/results.mdx: "on a failed call getData() is null" was true only for a request without a JSON answer (status 0, 408, 502, or an empty body). When Proxmox VE answers with an error (400, 500, 501) the answer has "data": null, so getData() is an empty node and it is get("name") on it that returns null.
• concepts/tasks.mdx: same wording where the UPID of a failed call is read.
• concepts/tasks.mdx: waitForTaskToFinish turns an interval of 0 or less into 500 ms, and a time shorter than the interval into the interval plus 5 seconds. It was not written.
• concepts/api-structure.mdx: "booleans are Boolean" was true only for the optional parameters. The required ones are primitive (int, boolean).

How it was checked

• The 65 Java blocks of the pages and of the README, taken as they are written, compile against the client 9.2.3. So do the 28 calls quoted in the text and in the tables.
• Every method, class and parameter named in the pages exists in the source.
• Read-only calls on a real cluster (Proxmox VE 8.4.21) for the behaviour described: unknown parameter (400), unknown endpoint (501), wrong password (login returns false, status 401), no credentials (401), a task log without limit (50 lines), a task that does not exist (PveResultException).
• npm run build: 14 pages.