[DOC] Enhancing the cached decorator documentation

[SolPier]

Just a bit of cleanup (I'm the previous author of this part of the documentation):

• some back-ticks were still needed to isolate the code in the plain text sections.
• I reformulated a few sentences as some words/expressions were redundant or needed clarification.
• changed the newValue to nextValue since the website syntax highlighting was misinterpreting it as a keyword (cf https://api.emberjs.com/ember/4.5/functions/@glimmer%2Ftracking/cached).

I'm happy to be able to set it right, I was a bit surprised (and ashamed) to see the mistakes when the text went online on the website :sweat_smile: .