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

doc/src/sgml/ddl.sgml

@@ -2280,9 +2280,16 @@ REVOKE ALL ON accounts FROM PUBLIC;
     <primary><type>aclitem</type></primary>
    </indexterm>
    The privileges that have been granted for a particular object are
-   displayed as a list of <type>aclitem</type> entries, where each
+   displayed as a list of <type>aclitem</type> entries, each having the
-   <type>aclitem</type> describes the permissions of one grantee that
+   format:
-   have been granted by a particular grantor.  For example,
+<synopsis>
+<replaceable>grantee</replaceable><literal>=</literal><replaceable>privilege-abbreviation</replaceable><optional><literal>*</literal></optional>...<literal>/</literal><replaceable>grantor</replaceable>
+</synopsis>
+   Each <type>aclitem</type> lists all the permissions of one grantee that
+   have been granted by a particular grantor.  Specific privileges are
+   represented by one-letter abbreviations from
+   <xref linkend="privilege-abbrevs-table"/>, with <literal>*</literal>
+   appended if the privilege was granted with grant option.  For example,
    <literal>calvin=r*w/hobbes</literal> specifies that the role
    <literal>calvin</literal> has the privilege
    <literal>SELECT</literal> (<literal>r</literal>) with grant option