Add documentation for spec.rules.host format

[zrdaley]

What this PR does / why we need it:

Wildcards/regex are not supported in the spec.rules.host field of an ingress definition.

Which issue this PR fixes : fixes #3534

cc: @ElvinEfendi @aledbf

[aledbf]

/approve

[aledbf]

waiting LGTM from @ElvinEfendi

[ElvinEfendi]

See #3534 (comment)

This is going to lead to more confusion since we support wildcard host everywhere else except session affinity. If we wanna go with changing docs then we should at least scope it for session affinity only.

[aledbf]

This is going to lead to more confusion since we support wildcard host everywhere else except session affinity.

No, this works just because nginx supports wildcard hostnames and we don't have additional validations to avoid this scenario. I am always open to "bend" the rules, but this one is not something I want to break, introducing the idea that wildcard hostnames are allowed.

The spec only allows wildcard hostnames in the TLS section, which is fine because of the nature of SSL certificates.

[ElvinEfendi]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi, zrdaley

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