Fix backticks in doc comments

[ijijn]

Hello!

Courtesy of clippy::doc_markdown, here is a collection of what I hope are fairly uncontroversial tweaks to the docs: adding/removing backticks for visual consistency. 😅

I left all mentions of SysEx alone because I see that they are consistently unticked and it's more of a concept than an explicit keyword (currently just an example plugin's name), but I could add those too if that's desired?

[robbert-vdh]

Thanks!