This commit contains assorted fixes for jsonpath documentation including:
grammar fixes, incorrect examples fixes as well as wording improvements.

Discussion: https://postgr.es/m/CAA-aLv4VVX%3Db9RK5hkfPXJczqaiTdqO04teW9i0wiQVhdKcqzw%40mail.gmail.com
Author: Liudmila Mantrova
Reviewed-by: Alexander Korotkov
Reported-by: Thom Brown