[doc] minor wording improvement in a couple of places

Hi

On these pages:

   https://www.postgresql.org/docs/current/fdw-callbacks.html
   https://www.postgresql.org/docs/current/tablesample-method.html

we have the phrase:

   "..., which see for additional details."

which strikes me as a bit odd. Suggested phrasing:

   "...; see this file for additional details."

Regards

Ian Barwick

-- 
Ian Barwick                   https://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services