Clarify that it is preferable to have unique class' serial name

[sandwwraith]

Fixes #3003

[pdvrieze]

This looks, good, but it is probably also a good idea to add the same/similar warnings to the various descriptor factories in kotlinx.serialization.descriptors.SerialDescriptors.kt

[sandwwraith]

We have "The resulting descriptor will be uniquely identified by the given [serialName], [typeParameters] and elements structure described in [builderAction] function." comment on buildClassSerialDescriptor and "[serialName] must be non-blank and unique." on PrimitiveSerialDescriptor. So only documentation for SerialDescriptor() requires an update.