English improvements for content guidelines

[Jayman2000]

This commit simplfies many sentences and rewords many sentences to make them sound more natural. In some places, this commit rewords sentences to make their structure more parallel. The goal of all of these changes is to make the content guidelines easier to understand.

Before this commit, the paragraph at the bottom of “Guidelines and principles” said pretty much the same thing that the paragraph at the bottom of “Limiting cognitive load” said. This commit combines those two paragraphs to reduce redundancy.