doc: more replacement of <literal> with something better

Bruce Momjian <bruce@momjian.us>

Commit: ca9cb940d23dc8869a635fa27a08e60837b17c07
Author: Bruce Momjian <bruce@momjian.us>
Date: 2016-08-25T01:11:44Z
Releases: 10.0
doc:  more replacement of <literal> with something better

Reported-by: Alexander Law

Author: Alexander Law

Backpatch-through: 9.6

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