doc: add commas after 'i.e.' and 'e.g.'
This follows the American format,
https://jakubmarian.com/comma-after-i-e-and-e-g/. There is no intention
of requiring this format for future text, but making existing text
consistent every few years makes sense.
Discussion: https://postgr.es/m/
20200825183619.GA22369@momjian.us
Backpatch-through: 9.5