Make Git subcommand documentation more consistent

[Denton-L]

In remote's documentation, in the Synopsis, the subcommands (add, remove, etc...) are underlined and Commands section, they are also underlined.

In submodule, the subcommands are not underlined in both they Synopsis and the Commands sections.

In worktree, the subcommands are underlined in the Synopsis but not in the Commands section.

I'm sure there are other Git commands that have subcommands and their documentation is probably all over the place as well. Anyway, we should look over this, find one style to stick with and standardise it for all the documentation.