minor #14609 [Serializer] Clarifies 'csv_headers' option (lajosthiel)

This PR was submitted for the 5.2 branch but it was squashed and merged into the 4.4 branch instead.

Discussion
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[Serializer] Clarifies 'csv_headers' option

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e22f015 [Serializer] Clarifies 'csv_headers' option

Author: javiereguiluz

components/serializer.rst

@@ -842,7 +842,14 @@ Option                  Description                                           De
 ``csv_escape_char``     Sets the escape character (at most one character)
 ``csv_key_separator``   Sets the separator for array's keys during its        ``.``
                         flattening
-``csv_headers``         Sets the headers for the data                         ``[]``, inferred from input data's keys
+``csv_headers``         Sets the order of the header and data columns
+                        E.g.: $this->serializer->serialize(
+                        ['c' => 3,'a' => 1,'b' => 2],
+                        'csv',
+                        ['csv_headers' => ['a', 'b', 'c']]);
+                        returns
+                        a,b,c
+                        1,2,3                                                 ``[]``, inferred from input data's keys
 ``csv_escape_formulas`` Escapes fields containg formulas by prepending them   ``false``
                         with a ``\t`` character
 ``as_collection``       Always returns results as a collection, even if only