fmt: document that unexported struct fields don't get the String/Error treatment

Fixes #17409.

Change-Id: Ib49ff4a467431b5c1e6637e5144979cf0bfba489
Reviewed-on: https://go-review.googlesource.com/31817
Reviewed-by: Martin Möhrmann <[email protected]>
Reviewed-by: Quentin Smith <[email protected]>

Author: rsc

src/fmt/doc.go

@@ -174,6 +174,9 @@
 	that type has a String method. Such pathologies are rare, however,
 	and the package does not protect against them.
 
+	When printing a struct, fmt cannot and therefore does not invoke
+	formatting methods such as Error or String on unexported fields.
+
 	Explicit argument indexes:
 
 	In Printf, Sprintf, and Fprintf, the default behavior is for each