Pushing btree opclass implementor's docs to the main SGML docs

I whined in <17947.1517767...@sss.pgh.pa.us> about the lack of any good place in the main documentation to explain how to write in_range() support functions for the upcoming window RANGE patch. What I would like to do, to create such a place, is to move the last section of src/backend/access/nbtre

Developer

The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/10/static/gin-intro.html Description: fff