Documentation request

Document text/x-shellscript-per-boot / text/x-shellscript-per-instance / text/x-shellscript-per-once in User-data formats.

What’s missing

• A subsection explaining the three “per-*” script frequencies:
  • per-boot – runs at every boot
  • per-instance – runs once per instance (first boot only)
  • per-once – runs exactly once ever
• Corresponding rows in the Content-Type table (alongside text/x-shellscript).
• A minimal multipart example showing all three MIME types.
• Pointers to the handler (cloudinit/handlers/shell_script_by_frequency.py), the modules (cc_scripts_per_boot, cc_scripts_per_instance, cc_scripts_per_once), and the on-disk script directories under /var/lib/cloud/scripts/.

Where to update

doc/rtd/explanation/format.rst (plus cross-link from doc/rtd/explanation/modules.rst).

Why

The feature is implemented, stable, and surfaced by cloud-init devel make-mime --list-types, yet undocumented. Proper docs will prevent user confusion and support load.

Related

#4197