Is there any API .xml file out there for NotePad++ which not only highlights API statements but also shows a list of auto-completetiong when e.g. typing f the following appears:
Notepad++ got builtin support for LUA and will identify the most common keywords/functions of lua (print,for,function,if etc) automatically. Most of the time it identifies this through the filetype of the file but you can manually set it by using the 'language' option in the menu.
WoW's API can vary alot between versions.
The functions, xml, globalstrings and their input-arguments can change for each version of WoW that is released; patch 5.0, 5.1, 5.2, etc.
Each time Blizzard releases a patch, addon-developers need to figure out what methods have been removed, added, and if their function-signatures have changed. Blizzard does not give much support for this. Comparisions between versions based on dumps of PTR code is what is mostly used (big thanks to those posting those diffs on the forums here btw), and community members then update the wiki's on different wow-websites that have more documentation listed.
So any such api.xml file would only be valid up until the next WoW patch, and it would require the author to review and update it between each wow-patch released.