docs(cli): clarify deno x JSR entry points

[lunadogbot]

Fixes denoland/docs#2845
Related to denoland/docs#3139
Closes bartlomieju/orchid-inbox#128

Summary

• Add deno x help text explaining that JSR packages use exports in deno.json or jsr.json, not package.json bin.
• Show how to expose a CLI subpath export and how to use a root export for command-only packages.

Verification

• cargo fmt --check -- cli/args/flags.rs

[lunadogbot]

node_compat::parallel::test-cluster-send-deadlock.js timed out after 10000ms; this is unrelated to the cli/args/flags.rs docs change. Re-running.

[lunadogbot]

@bartlomieju CI is green on this upstream source PR. This is ready for review.

[fibibot]

LGTM. The help text accurately explains how resolves JSR exports and gives concrete root/subpath examples. CI is green. @bartlomieju this looks ready to merge.

[fibibot]

LGTM. The help text accurately explains how deno x jsr: resolves JSR exports and gives concrete root/subpath examples. CI is green. @bartlomieju this looks ready to merge.

[fibibot]

LGTM. The help text clearly documents how JSR entry points are resolved for deno x and CI is green.\n\n@bartlomieju this is ready to merge.

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