docs(api): add GLAB_DEBUG_HTTP example for debugging requests

What does this MR do?

Adds an example to glab api --help showing GLAB_DEBUG_HTTP=1 for debugging the request/response that the command is sending. The env var already exists and renders full headers + body via debugTransport, but it wasn't discoverable from the command's own help output.

The example uses a --field payload (the situation that motivated the issue) so users searching for "how do I see what's actually being sent" land on the same shape that prompted the question.

No new flag, no new behavior — just surfacing what's already there.

Resolves #8351 (closed).