doc: Make SQL command names in the catalog documentation links

In passing, fix the initdb references to be <application> rather than
<command>, which is what we normally use.

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://www.postgresql.org/message-id/flat/87mu5xqc11.fsf@wibble.ilmari.org