docs: [SITE-5883][SITE-5884] Document + announce WordPress update-notice dismiss & hide controls

[mehta-asim]

Summary

• Document how to control the Pantheon WordPress core update notice in Suppress WordPress Admin Notice: dismiss it (per-user, persists until a newer version — SITE-5883), and hide it via CSS (#pantheon-update-notice), the pantheon_show_update_notice filter, or the PANTHEON_SHOW_UPDATE_NOTICE constant (SITE-5884).
• Correct the existing DISABLE_PANTHEON_UPDATE_NOTICES description. The old text said it "only disables the text and notice in the WordPress admin." That is inaccurate: the constant stops the plugin's update-notice handling from loading entirely, so WordPress's own default update nag can appear instead. Add a note distinguishing the cosmetic hide options from this subsystem-level opt-out.
• Add a release note announcing the dismiss + hide controls (src/source/releasenotes/2026-07-15-wordpress-update-notice-controls.md).

Changes

File	Change
src/source/content/core-updates.md	Rewrote the "Suppress WordPress Admin Notice" section: dismiss + CSS/filter/constant hide options + corrected DISABLE_ behavior in an Alert
src/source/releasenotes/2026-07-15-wordpress-update-notice-controls.md	New release note (categories: new-feature, wordpress)

Context

Companion to the plugin work: pantheon-systems/pantheon-mu-plugin#119 (SITE-5884: CSS hook + pantheon_show_update_notice filter + PANTHEON_SHOW_UPDATE_NOTICE constant) and pantheon-systems/pantheon-mu-plugin#121 (SITE-5883: dismissible notice), both merged.

The corrected DISABLE_ behavior was verified against the mu-plugin source and WordPress core:

• The constant gates whether inc/pantheon-updates.php loads (pantheon.php). That file is the only place that removes WordPress's native update_nag and forces WP_AUTO_UPDATE_CORE = false.
• WordPress core registers update_nag on admin_notices (priority 3) by default and renders it whenever a core update is available.
• The plugin never suppresses core-update detection, so with the file unloaded the native nag is no longer removed and can render.

Blocked on release

These features are unreleased. They ship in pantheon-mu-plugin 1.5.7 (release cut by pantheon-systems/pantheon-mu-plugin#123) and reach sites when that version is bundled into the WordPress upstream. Hold this PR until then, and set the release note's published_date / published_at to the actual ship date before merging.

Test plan

• Preview renders the "Suppress WordPress Admin Notice" section (dismiss + three hide options + the Alert).
• Preview renders the new release note.
• Set the release note date to the real ship date.
• Merge only after pantheon-mu-plugin 1.5.7 ships to the WordPress upstream.

References

• Jira: SITE-5883, SITE-5884
• Plugin PRs: feat(updates): [SITE-5883] Add dismiss option to WordPress update notice pantheon-mu-plugin#121 (dismiss), feat(updates): [SITE-5884] Add CSS hook and filter to hide WordPress update notice pantheon-mu-plugin#119 (hide options)
• Release PR: chore: release 1.5.7 pantheon-mu-plugin#123 (1.5.7)

[pantheon-site-integration]

Successfully created multidev environment: pr-10166

[pantheon-site-integration]

Successfully created multidev environment: pr-10166

[pantheon-site-integration]

Successfully created multidev environment: pr-10166