Does brevity refer to speech and writing or the content itself?

Well, if you look carefully at the definition of brevity, you will see that it does refer to content, too, in the sense that content is expressed into words. M-W defines brevity

especially : shortness or conciseness of expression

However, if you try to characterise a document, I think a brief document does not really refer to lack of redundant details. I would rather say this is a concise document. So I think that

I have placed emphasis on conciseness

is more recommended.

The same dictionary, M-W, shows that conciseness is more accurate here, as it means:

marked by brevity of expression or statement : free from all elaboration and superfluous detail

• a concise report