add docs

Change-Id: Ic68413b71d1f605d131c7f909839e565198f074b

Author: callthingsoff

doc/next/6-stdlib/99-minor/encoding/xml/69857.md

@@ -0,0 +1,6 @@
+When marshaling, a struct field with the new `omitzero` option in the struct field
+tag will be omitted if its value is zero. If the field type has an `IsZero() bool`
+method, that will be used to determine whether the value is zero. Otherwise, the
+value is zero if it is [the zero value for its type](/ref/spec#The_zero_value).
+If both `omitempty` and `omitzero` are specified, the field will be omitted if the
+value is either empty or zero (or both).

src/encoding/xml/marshal.go

@@ -60,6 +60,13 @@ const (
 //     if the field value is empty. The empty values are false, 0, any
 //     nil pointer or interface value, and any array, slice, map, or
 //     string of length zero.
+//   - a field with a tag including the "omitzero" option is omitted
+//     if the field has a zero value, according to rules:
+//     1) If the field type has an "IsZero() bool" method, that will be used to
+//     determine whether the value is zero.
+//     2) Otherwise, the value is zero if it is the zero value for its type.
+//     If both "omitempty" and "omitzero" are specified, the field will be omitted
+//     if the value is either empty or zero (or both).
 //   - an anonymous struct field is handled as if the fields of its
 //     value were part of the outer struct.
 //   - an anonymous struct field of interface type is treated the same as having