titles dont have documentation

The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/10/static/sql-syntax.html Description: I didnt see documentation in postgresql title

remove endterms and title ids

The attached patch removes endterm attributes from xref elements and the associated id attributes on title elements. Older documentation toolchains could not automatically generate text for xrefs into refentries, so we had to use the endterm attribute to tell it which text to create. This is no l