use "class-string" where appropriate in relations (#55074)

we'll use the more accurate "class-string" type for "morphClass" references.

also specify the generic type where appropriate

Author: browner12

src/Illuminate/Database/Eloquent/Relations/MorphOneOrMany.php

@@ -25,7 +25,7 @@ abstract class MorphOneOrMany extends HasOneOrMany
     /**
      * The class name of the parent model.
      *
-     * @var string
+     * @var class-string<TRelatedModel>
      */
     protected $morphClass;
 
@@ -159,7 +159,7 @@ public function getMorphType()
     /**
      * Get the class name of the parent model.
      *
-     * @return string
+     * @return class-string<TRelatedModel>
      */
     public function getMorphClass()
     {

src/Illuminate/Database/Eloquent/Relations/MorphPivot.php

@@ -18,7 +18,7 @@ class MorphPivot extends Pivot
      *
      * Explicitly define this so it's not included in saved attributes.
      *
-     * @var string
+     * @var class-string
      */
     protected $morphClass;
 
@@ -100,7 +100,7 @@ public function setMorphType($morphType)
     /**
      * Set the morph class for the pivot.
      *
-     * @param  string  $morphClass
+     * @param  class-string  $morphClass
      * @return \Illuminate\Database\Eloquent\Relations\MorphPivot
      */
     public function setMorphClass($morphClass)

src/Illuminate/Database/Eloquent/Relations/MorphToMany.php

@@ -25,7 +25,7 @@ class MorphToMany extends BelongsToMany
     /**
      * The class name of the morph type constraint.
      *
-     * @var string
+     * @var class-string<TRelatedModel>
      */
     protected $morphClass;
 
@@ -212,7 +212,7 @@ public function getQualifiedMorphTypeName()
     /**
      * Get the class name of the parent model.
      *
-     * @return string
+     * @return class-string<TRelatedModel>
      */
     public function getMorphClass()
     {