Document optional placeholders in style guide

[kbdharun]

Thanks for highlighting the places this is used, we support usage like this to highlight file extensions, etc inside a placeholder i.e. {{path/to/source.tar[.gz|.bz2|.xz]}} but for other places like this [{{PATH}}] I would suggest adding it to the description instead (and removing it) as there are a few tldr clients which allows passing an example directly on the host by filling in the placeholder arguments and this would break their output (Fun fact: some of the older tldr pages tutorials on YouTube suggests these kinds of clients, but I am neither familiar with them nor sure how popular they are).

I think it would be good to document this usage in the style guide and also fix the existing pages having this behavior.

Originally posted by @kbdharun in #12368 (comment)

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Creating this issue, as a reminder for myself to work on this later.