Review of the glossary

Add some more terms, clarify some definitions, remove redundant terms,
move a couple of terms to keep alphabetical order.

Co-authored-by: Jürgen Purtz <[email protected]>
Co-authored-by: Erik Rijkers <[email protected]>
Co-authored-by: Laurenz Albe <[email protected]>
Discussion: https://postgr.es/m/[email protected]