In the manual under "Graphic Parameters: Line Width, Line Cap, and Line Join" on /tikz/line cap, the sample output goes round, butt, rect down the page but the source code goes the opposite way rect, butt, round.
It could help clarity if the source code was the same order as the output, for ease of cutting and pasting when you see the style you like in the image.
round isn't likely to be confused, but I had to check twice which is which between butt and rect.
Diff:
Done. Thank you for reporting.