Add the "Plugin API" page to the "Related topics" list of hook_PLUGIN_TYPE_plugin_alter() documentation

Drupal core has documentation for these hooks.

• hook_search_plugin_alter()
• hook_element_plugin_alter()
• hook_display_variant_plugin_alter()

Those documentation pages show, in Related topics, a link to Hooks. Since they are hooks related to plugins, those pages should also contain a link to Plugins.

This task is essentially the following.

• In core/modules/search/search.api.php, find the documentation comment for hook_search_plugin_alter() and add a @ingroup plugin_api at the end of the documentation comment
• In core/lib/Drupal/Core/Render/theme.api.php, find the documentation comment for hook_element_plugin_alter() and add a @ingroup plugin_api at the end of the documentation comment
• In core/core.api.php, find the documentation comment for hook_display_variant_plugin_alter() and add a @ingroup plugin_api at the end of the documentation comment