The short version is for typing on the fly, and the long version is for scripts, they should have identical output.
The full thorough documentation should be in man, and/or info.
I agree with your point that most flags should generally treat short versions as exact aliases to long flags, but I just think that a convention that treats -h and --help as concise vs long is 100% reasonable. The distinction is often breadth vs depth.
Having them be different could cause someone to look at -h, and not even know about --help. Or if someone writes a script parsing the output of -h for some reason, someone else might come along and change it to --help expecting it to be the same thing.
This convention existed before clap came into being, but I don't recall when I first saw it. I have been using the command line for just shy of 40 years across various operating systems.
You can just make a `--help-all` (or whatever word you want to use), imo the `--help-all` command doesn't need a short equivalent because it's not something you'd frequently use.
It's not cosmetic. Uniform help is a way to not let agents hallucinate. Otherwise you end up with invalid commands, or worse, silent ones that go through without doing anything at all, or go totally wrong.