Fixing the tsctl documentation

[Aevyz]

So after my Issue from yesterday #3436, I decided to take a look at the entire document to see if I can find further inconsistencies. I'll try to fix as many of them as possible, but for some I might need to defer to someone who developed the tsctl python script.

Issues Found:

• [x] Docs refer to drop_db but should be drop-db
• [ ] grant-user is not documented
• [ ] import-search-templates is not documented
• [ ] list-group-members is not documented (docs refer to a --showmembership flag, which I do not believe exists anymore)
• [ ] list-sketches is only referred to under tsctl, does not have own subpoint
• [x] search_template was removed, remains in docs
• [x] import was removed, remains in docs
• [x] add_index was removed, remains in docs

I'll continue updating this issue if I find additional points that need resolving.