Closes #8031.

This PR fixes a number of problems (some objective, some aesthetic) in [simd.syn]:

• Comments with references to subsequent sections should not capitalize the section name
• vec should be \tcode{vec}
• The two-space indentation in some places was missing
• Some code just needs to be reflown because parameters are misaligned and other things are broken

Aesthetics are also improved a bit. While it's not objectively wrong to do so, putting countl_one on one line and countl_zero onto two because of a single-character difference is ugly. It substantially improves aesthetics if we make it a bit more consistent.