Update availability of PlantUML

What does this MR do?

As part of !91639 (merged) we updated doc/administration/integration/plantuml.md but I had a hunch we had more mentions of PlantUML. Specifically, I thought I remembered seeing a mention of it on the Markdown page. A smidge of grepping later, and I found a few other places that needed minor updates as part of the change @astarovoytov and @marcel.amirault worked on together:

• doc/api/settings.md
• doc/user/asciidoc.md
• doc/user/markdown.md

I'll route these changes over to Marcel for review / edit / merge. These are wordsings from late on a Friday afternoon, and the odds of me getting it right on the first try aren't great...

Related issues

• Related to Add note for SaaS clients using PlantUML integr... (!91639 - merged)

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