Issue Description:

If there is a non-link item in a "See Also" list or Topics list in a Swift source file, DocC will emit a warning: "Only links are allowed in task group list items" without a source location.

This makes locating the cause of the warning incredibly difficult. The only way I was able to discover where these things were coming from is by building my own version of DocC with a bunch of "dump()" calls thrown around.

This revealed that DocumentCurator was unable to find a document location for the node reference here:

Steps to reproduce:

Add an item to a "See Also" list or "Topics" list which is just plain text (not a link), and build documentation. DocC will emit a warning with no location.

/// A thing
///
/// ## Topics
///
/// ### Things about Things
///
/// - bar
///
struct Foo {
}