Adjust some glossary terms

Mostly in response to Jürgen Purtz critique of previous definitions,
though I added many other changes.

Author: Álvaro Herrera <[email protected]>
Reviewed-by: Jürgen Purtz <[email protected]>
Reviewed-by: Justin Pryzby <[email protected]>
Reviewed-by: Erik Rijkers <[email protected]>
Discussion: https://postgr.es/m/[email protected]

Author: alvherre