encoding/xml: add Marshal doc about name conflicts

The docs for xml.Marshal state that the XML elements name is derived
from one of five locations in a specific order of precedence, but does
not mention that if the field is a struct type and has its name defined
in a tag and in the types XMLName field that an error will occur. This
is documented in the structFieldInfo function but not in the function
documentation, and the existing docs in Marshal are misleading without
this behavior being discussed.

Fixes #18564

Change-Id: I29042f124a534bd1bc993f1baeddaa0af2e72fed
Reviewed-on: https://go-review.googlesource.com/76321Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/xml/marshal.go

@@ -67,6 +67,9 @@ const (
 // parent elements a and b. Fields that appear next to each other that name
 // the same parent will be enclosed in one XML element.
 //
+// If the XML name for a struct field is defined by both the field tag and the
+// struct's XMLName field, the names must match.
+//
 // See MarshalIndent for an example.
 //
 // Marshal will return an error if asked to marshal a channel, function, or map.