[docs] Client certificates add note about rejecting all the certificates

[mxschmitt]

• client certificates are only active if > 0 are passed
• if you want to reject client certificates, you need to pass a 'dummy client certificate' so the server is active with a host which doesn't match at all.
• adjust the documentation, so that it surfaces that.

• playwright/docs/src/api/params.md

  Line 549 in 4a8438e

  ## context-option-clientCertificates