Clarify precautions around hooks which dispatch on install/upgrade

Author: cameron1729

docs/apis/core/hooks/index.md

@@ -296,9 +296,10 @@ Any guidelines for callback priority should be described in hook descriptions if
 
 :::caution
 
-Callbacks _are executed during system installation and all upgrades_, the callback
-methods must verify the plugin is in correct state. Often the easiest way is to
-use function during_initial_install() or version string from the plugin configuration.
+Unlike lib.php based plugin callbacks, certain hooks _are dispatched during system installation and upgrade_ (for example `before_http_headers`). Callback
+methods for such hooks must take extra care to ensure the plugin is properly initialised, as (among other things) on install _the site's database is not initialised_.
+The function during_initial_install() or version string from the plugin configuration may be used to conditionally make DB queries or use API functions in such cases.
+Failure to do so may render the web install/upgrade page unusable.
 
 :::