doc: keep first level section header in upper case

doc: keep first level section header in upper case

When formatted as a man page, 1st section header is always in upper
case even if we write it otherwise. Make all 1st section headers
uppercase to keep it close to the final output.

This does affect html since case is kept there, but I still think it's
a good idea to maintain a consistent style for 1st section headers.

Some sections perhaps should become second sections instead, where
case is kept, and for better organization. I will update if anyone has
suggestions about this.

While at there I also make some header more consistent (e.g. examples
vs example) and fix a couple minor things here and there.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>