Add an advanced section to the search help page

[Daniel Zoller]

This section should cover the possibility to search in specific fields. For the name of the fields see https://bitbucket.org/bibsonomy/bibsonomy/src/a903b693790093ab697846f392b194622e4bad7d/bibsonomy-search-elasticsearch/src/main/java/org/bibsonomy/search/es/ESConstants.java?at=default&fileviewer=file-view-default

Add the workaround of https://bitbucket.org/bibsonomy/bibsonomy/issues/2729/allow-users-to-query-posts-with-a-tag-or to the section

Check if we link to the search help page from the URL-Syntax help page.