Document that len does not include skipped struct fields

[Marcono1234]

At least this seems to be how serde_derive behaves:

serde/serde_derive/src/ser.rs

Lines 335 to 341 in 3f43fca

	.map(|field| match field.attrs.skip_serializing_if() {
	None => quote!(1),
	Some(path) => {
	let field_expr = get_member(params, field, &field.member);
	quote!(if #path(#field_expr) { 0 } else { 1 })
	}
	})

Explicitly documenting this makes it easier for custom Serializer implementations to know what to expect from the len value, and makes it clearer for custom Serialize implementations to know what to provide as value.

[dtolnay]

Thank you!