Reviewers on apps#3751 (support compound command and resetcause-based
triggers) asked for documentation of the new features. Add two
sections to the nxinit doc:
- "Built-in Properties": describes the sys.boot.reason property set
by NXInit at startup from BOARDIOC_RESET_CAUSE, its two value forms
(hardware cause with numeric subreason, or software reset reason
string), and behavior when CONFIG_BOARDCTL_RESET_CAUSE is disabled
or the boardctl() call fails.
- "Compound Commands": describes the && / || short-circuit semantics
for chaining commands on a single action line, including quoting
behavior.
Assisted-by: GitHubCopilot:claude-sonnet-5
Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>