Add additional headers to FlashMessage page (#1944)

* Add additional headers to FlashMessage page

Previously, a large content block was under "Message severities"
which was primarily not related to the severities. Now, additional
headers have been added for the sections.

* Fix typos

Co-authored-by: Lina Wolf <48202465+linawolf@users.noreply.github.com>

* Fix typos

Co-authored-by: Lina Wolf <48202465+linawolf@users.noreply.github.com>

* Use FlashMessage

Co-authored-by: Lina Wolf <48202465+linawolf@users.noreply.github.com>

* Use FlashMessage

Co-authored-by: Lina Wolf <48202465+linawolf@users.noreply.github.com>

* Use FlashMessage

Co-authored-by: Lina Wolf <48202465+linawolf@users.noreply.github.com>

* Fix header underlines

* Fix conflict resolution

Co-authored-by: Lina Wolf <48202465+linawolf@users.noreply.github.com>

Documentation/ApiOverview/FlashMessages/Index.rst

@@ -37,6 +37,9 @@ The different severity levels are described below:
 Flash messages API
 ==================
 
+Instantiate a flash message
+---------------------------
+
 Creating a flash message is achieved by simply instantiating an object
 of class :php:`\TYPO3\CMS\Core\Messaging\FlashMessage`
 
@@ -84,6 +87,14 @@ The severity is defined by using class constants provided by
 
 *  :php:`FlashMessage::ERROR` for errors
 
+The fourth parameter passed to the constructor is a flag that
+indicates whether the message should be stored in the session or not (the
+default is not). Storage in the session should be used if you need the
+message to be still present after a redirection.
+
+Add a flash message to the queue
+--------------------------------
+
 In backend modules you can then make that message appear on top of the
 module after a page refresh or the rendering of the next page request
 or render it on your own where ever you want.
@@ -138,11 +149,10 @@ as top-right notifications, instead of inline:
     );
     $notificationQueue->enqueue($flashMessage);
 
-The recommend way to show flash messages is to use the Fluid Viewhelper :html:`<f:flashMessages />`.
-This Viewhelper works in any context because it use the :php:`FlashMessageRendererResolver` class
+The recommended way to show flash messages is to use the Fluid ViewHelper :html:`<f:flashMessages />`.
+This ViewHelper works in any context because it use the :php:`FlashMessageRendererResolver` class
 to find the correct renderer for the current context.
 
-
 .. _flash-messages-renderer:
 
 Flash messages renderer