Improve documentation for when submodule references are stale

[theunrepentantgeek]

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Describe the current behavior

A common error is to fail to update our git submodules after doing a pull, which can result in the local working folder being stale. This in turn can result in errors like this one when the generator is run:

E0211 14:19:54.561995   27841 gen_kustomize.go:111] Error during code generation:
failed during pipeline stage 23/57: 
Verify there are no ErroredType's containing errors: 
"github.com/Azure/azure-service-operator/v2/api/network/v1alpha1api20180901/PrivateDnsZone" has property "Status" with errors:
"missing status information for github.com/Azure/azure-service-operator/v2/api/network/v1alpha1api20180901/PrivateDnsZones"

(Message has been formatted for readability.)

Even if this particular message isn't caused by stale submodule references, this has happened frequently in the past.

Describe the improvement

We need to improve our documentation so that someone searching our repo for this error can find guidance.

[matthchr]

@porges is working on #2084 which might end up changing the format of this particular error (and also changes what submodules we even need). So it might make sense to do this after that merges.

[theunrepentantgeek]

We should still do this.

[matthchr]

Probably should still do this