Document error conditions for Command::{spawn, output, status}

[sanidhyasin]

Fixes #150361.

Command::spawn, Command::output, and Command::status all return io::Result, but the docs never explain why they can fail, and they do not mention the (easy to get wrong) distinction between a failure to spawn and a child that runs but exits unsuccessfully or is killed by a signal.

This adds an # Errors section to each method:

• spawn describes the common reasons spawning fails — the program not being found, lacking permission to execute it (e.g. not executable, or blocked by a policy such as seccomp), and the OS being unable to create the process due to resource exhaustion. It also clarifies that an error is only returned for failures while spawning; once the child has started, anything that happens to it (including signals) is reported through its ExitStatus.
• output and status refer back to spawn for the spawn failures, and explicitly note that a child which exits unsuccessfully or is terminated by a signal is not an error — they still return Ok, with the outcome reflected in the resulting ExitStatus.

The error conditions and the signal/ExitStatus behavior were confirmed by @bjorn3 in the issue discussion.

Docs-only change; no code blocks were added or modified, and all intra-doc links (io::Error, ExitStatus, Output, spawn) follow link patterns already used in this module.

r? libs

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