More explicit column index as string

Fixes #951

src/PhpSpreadsheet/Worksheet/AutoFilter/Column.php

@@ -101,7 +101,7 @@ class Column
     }
 
     /**
-     * Get AutoFilter Column Index.
+     * Get AutoFilter column index as string eg: 'A'.
      *
      * @return string
      */
@@ -111,7 +111,7 @@ class Column
     }
 
     /**
-     * Set AutoFilter Column Index.
+     * Set AutoFilter column index as string eg: 'A'.
      *
      * @param string $pColumn Column (e.g. A)
      *

src/PhpSpreadsheet/Worksheet/Column.php

@@ -40,7 +40,7 @@ class Column
     }
 
     /**
-     * Get column index.
+     * Get column index as string eg: 'A'.
      *
      * @return string
      */

src/PhpSpreadsheet/Worksheet/ColumnDimension.php

@@ -42,7 +42,7 @@ class ColumnDimension extends Dimension
     }
 
     /**
-     * Get ColumnIndex.
+     * Get column index as string eg: 'A'.
      *
      * @return string
      */
@@ -52,7 +52,7 @@ class ColumnDimension extends Dimension
     }
 
     /**
-     * Set ColumnIndex.
+     * Set column index as string eg: 'A'.
      *
      * @param string $pValue
      *