DOC Document new methods on GridFieldFilterHeader (#597)

en/08_Changelogs/5.4.0.md

@@ -41,6 +41,7 @@ SilverStripe\ORM\FieldType\DBPolymorphicForeignKey:
   - [`ArrayLib::insertAfter()`](api:SilverStripe\ORM\ArrayLib::insertAfter()) for inserting a value after another value in an array
 - A new [`DBDatetime::getTimeBetween()`](api:SilverStripe\ORM\FieldType\DBDatetime::getTimeBetween()) method has been added. This method returns the amount of time that has passed between two `DBDateTime` objects as a human-readable string.
 - A new [`AbstractQueuedJob::getQueue()`](api:Symbiote\QueuedJobs\Services\AbstractQueuedJob::getQueue()) static method has been added to get the correct queue constant from a given string or int.
+- New [`GridFieldFilterHeader::setPlaceHolderText()`](api:SilverStripe\Forms\GridField::GridFieldFilterHeader::setPlaceHolderText()) and [`GridFieldFilterHeader::getPlaceHolderText()`](api:SilverStripe\Forms\GridField::GridFieldFilterHeader::getPlaceHolderText()) methods have been added which provide a way to override the `GridFieldFilterHeader` search field placeholder text if the dynamically generated text doesn't suit your use case.
 
 ## API changes