Improve clarity of channel notification limit messages (#32160)

* Improve clarity of channel notification limit messages

- Update user-facing messages to explain performance reasoning and provide guidance
- Enhance admin setting descriptions to be more informative about user experience  
- Change admin setting title to better reflect purpose

Fixes #32159

Co-authored-by: Sven Hüster <svelle@users.noreply.github.com>

* Update webapp/channels/src/i18n/en.json

Co-authored-by: Matthew Birtch <mattbirtch@gmail.com>

* Update server/i18n/en.json

Co-authored-by: Matthew Birtch <mattbirtch@gmail.com>

* Update server/i18n/en.json

* Update server/i18n/en.json

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: Sven Hüster <svelle@users.noreply.github.com>
Co-authored-by: Carrie Warner (Mattermost) <74422101+cwarnermm@users.noreply.github.com>
Co-authored-by: Matthew Birtch <mattbirtch@gmail.com>

server/i18n/en.json

@@ -2634,15 +2634,15 @@
   },
   {
     "id": "api.post.disabled_all",
-    "translation": "@all has been disabled because the channel has more than {{.Users}} users."
+    "translation": "@all notifications are disabled in channels with more than {{.Users}} users. Contact your system administrator for more information."
   },
   {
     "id": "api.post.disabled_channel",
-    "translation": "@channel has been disabled because the channel has more than {{.Users}} users."
+    "translation": "@channel notifications are disabled in channels with more than {{.Users}} users. Contact your system administrator for more information."
   },
   {
     "id": "api.post.disabled_here",
-    "translation": "@here has been disabled because the channel has more than {{.Users}} users."
+    "translation": "@here notifications are disabled in channels with more than {{.Users}} users. Contact your system administrator for more information."
   },
   {
     "id": "api.post.do_action.action_id.app_error",