Doc: Improve description of connection strings with Percent-encoding

Clarify the description related to the use of characters which can be
encoded, and add an example.

Author: Jobin Augustine
Reviewed-by: Peter Eisentraut, Alvaro Herrera, Heikki Linnakangas,
Michael Paquier, Alex Shulgin
Discussion: https://postgr.es/m/CANaTPsrYgSgE2fuj3=4x=Jmx1c+NgkEDzftNknZbrMuqL+aBhQ@mail.gmail.com