Search isn't showing all the results I'd expect

**Is your suggestion related to a problem? Please describe.**
Some users are reporting that, when trying to use the search, they're [surprised/frustrated](https://github.com/vector-im/element-web/issues/14617#issuecomment-663125578) to find it's just searching rooms/DMs they're already a member of.
As the UI and copy makes it look like a global search, and not a filter, it would be reasonable to understand the issue.

**Describe the solution you'd like**
An easily accessible global search that searches more than just rooms I'm in.

**Describe alternatives you've considered**
Cmd + F to search for specific names. Making the explore button more obvious might help but that's not a solution to this problem, because users are not thinking they need to decide between rooms they're in and rooms they're not in at this stage.

**Additional context**
People's expectations may have been set by other products they use - where prominent searches are for quickly finding anything - Gmail, Github, Facebook, etc etc.