encoding/csv: clarify that TrimLeadingSpace can trim the delimiter

Fixes #14464

Change-Id: Iafc21641cca7d35b7a5631cfc94742ee8e7d5042
Reviewed-on: https://go-review.googlesource.com/19861Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/csv/reader.go

@@ -99,6 +99,8 @@ var (
 // non-doubled quote may appear in a quoted field.
 //
 // If TrimLeadingSpace is true, leading white space in a field is ignored.
+// If the field delimiter is white space, TrimLeadingSpace will trim the
+// delimiter.
 type Reader struct {
 	Comma            rune // field delimiter (set to ',' by NewReader)
 	Comment          rune // comment character for start of line