Stricter typing for mergeCells

Closes #1494

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Fix incorrect behaviour when saving XLSX file with drawings [#1462](https://github.com/PHPOffice/PhpSpreadsheet/pull/1462),
 - Fix Crash while trying setting a cell the value "123456\n" [#1476](https://github.com/PHPOffice/PhpSpreadsheet/pull/1481)
 - Improved DATEDIF() function and reduced errors for Y and YM units [#1466](https://github.com/PHPOffice/PhpSpreadsheet/pull/1466)
+- Stricter typing for mergeCells [#1494](https://github.com/PHPOffice/PhpSpreadsheet/pull/1494)
 
 ### Changed
 

src/PhpSpreadsheet/Worksheet/Worksheet.php

@@ -187,7 +187,7 @@ class Worksheet implements IComparable
     /**
      * Collection of merged cell ranges.
      *
-     * @var array
+     * @var string[]
      */
     private $mergeCells = [];
 
@@ -1747,7 +1747,7 @@ class Worksheet implements IComparable
     /**
      * Get merge cells array.
      *
-     * @return array[]
+     * @return string[]
      */
     public function getMergeCells()
     {
@@ -1758,6 +1758,8 @@ class Worksheet implements IComparable
      * Set merge cells array for the entire sheet. Use instead mergeCells() to merge
      * a single cell range.
      *
+     * @param string[] $pValue
+     *
      * @return $this
      */
     public function setMergeCells(array $pValue)