Using highlighting you can return the relevant parts of a field and using highlighted markers, indicate why a document matched

var result = this._client.Search<ElasticSearchProject>(s => s
    .Query(q => q
        .QueryString(qs => qs
            .OnField(e => e.Content)
            .Query("null or null*")
    .Highlight(h => h
        .OnFields(f => f
            .OnField(e => e.Content)

Please take note that this won’t alter the contents of the results .Documents but the results will have a separate bucket that contains each highlight result(s) for each hit on result.Highlights.