Doc: specify aclitem syntax more clearly.
The previous wording here relied solely on an example to explain
aclitem output format. Add an actual syntax synopsis and
explanation of the elements to make it clearer.
David Johnston and Tom Lane, per gripe from Eugen Konkov.
Discussion: https://postgr.es/m/
170326116972.
1876499.
18357820037829248593@wrigleys.postgresql.org