doc: more replacement of <literal> with something better

Bruce Momjian <bruce@momjian.us>

Commit: 1414d490f7eee23bf3544953a47340e9b54fcf29
Author: Bruce Momjian <bruce@momjian.us>
Date: 2016-08-25T01:11:44Z
Releases: 9.6.0
doc:  more replacement of <literal> with something better

Reported-by: Alexander Law

Author: Alexander Law

Backpatch-through: 9.6

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