Problems:
- Miss some entries in `vim_diff.txt` and `index.txt`.
- I want to learn about Vim register, but when I type `:h register`, it
shows sponsor information instead. Note that unlike Nvim, Vim has a
separate session for `*register*`
Solution:
- Add missing commands to `index.txt`, `vim_diff.txt`
- Remove tag `register` from `index.txt`
d88bebfbc7
