Document spec search filter

[bahmutov]

I'm submitting a...

[ ] Bug report
[x] Content update
[ ] Process update (build, deployment, ... )

We are about to release cypress-io/cypress#8007 where the specs name filter affects the "Run all specs" button. It would be nice to document it. I am thinking a new section before https://docs.cypress.io/guides/core-concepts/writing-and-organizing-tests.html#Watching-tests would be a good place