Document wildcard support in user search

## Description of documentation change
Just learned today that wildcards are supported in user search and tought it would be nice to add this information in the documentation.

## Existing documentation section (if applicable)
https://dnndocs.com/content/guides/tutorials/securing-your-site/user-accounts/about-user-accounts/index.html

## Proposed area for new documentation (if applicable)
A new subpage called "Searching for users" that documents how the search works, IE right now I learned we can use wildcards * that I was not aware of.

## Additional context
I am working on a bug with the search and the "show" dropdown in that area so I may add some more info if I have more findings worth documenting.