Docsite: fix typos, small clarity change

[cyniphile]

Hi Team! This is a small PR to clean up a few things I saw while reading up on the Under-the-hood section.

• Fixes some small typos
• Fixes rendering of two equations ($$ need to be inline)
• Moves "factors are currently length 1" explanation further up in the document so subsequent use of $abs(f_u)$ makes a bit more sense (otherwise I'd probably expect $||f_u||$ notation)

[CLAassistant]

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