tls user guide --default-ssl-certificate clarification

[multi-io]

Evidently the --default-ssl-certificate option is used not only for the catch-all server, but also for all ingress tls: sections that don't have a secretName option. This doesn't seem to be documented anywhere, hence this change.

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

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[ElvinEfendi]

/ok-to-test

[ElvinEfendi]

/lgtm

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