Update docs for setHeading
Resolves #472
Adds JavaDoc for PVector#setHeading() and the associated keywords.txt entry, generated using the instruction at https://github.com/processing/processing-website?tab=readme-ov-file#keywordstxt. This allows the Syntax Highlighting and Open Content Reference context menu item to work.
(Note: Open Content Reference will open missing page until https://github.com/processing/processing-website/pull/621 is merged and properly deployed.)
(Note 2: Also updates the documentation for PVector#heading() to be more explanatory, inspired by https://p5js.org/reference/p5.Vector/heading, so that JSON might also require regeneration. (Done) )
Adds more missing keywords to the keywords.txt file. (Generated via keywords.txt update routine)