[3.12] docs: clarify csv.DictReader's treatment of the first data row (pythonGH-118549) (python#118555)

(cherry picked from commit 9d67b72)

Co-authored-by: Ned Batchelder <[email protected]>

Author: miss-islington

Doc/library/csv.rst

@@ -156,8 +156,10 @@ The :mod:`csv` module defines the following classes:
 
    The *fieldnames* parameter is a :term:`sequence`.  If *fieldnames* is
    omitted, the values in the first row of file *f* will be used as the
-   fieldnames.  Regardless of how the fieldnames are determined, the
-   dictionary preserves their original ordering.
+   fieldnames and will be omitted from the results. If
+   *fieldnames* is provided, they will be used and the first row will be
+   included in the results.  Regardless of how the fieldnames are determined,
+   the dictionary preserves their original ordering.
 
    If a row has more fields than fieldnames, the remaining data is put in a
    list and stored with the fieldname specified by *restkey* (which defaults