ResourceSnippet does not support PayloadSubsectionExtractor

Hello,

When migrating from vanilla Spring REST Doc to this plugin, we encountered a problem with PayloadSubsectionExtractor.
It is not used at all.
Here is our exemple:

```
document("example",
RequestDocumentation.pathParameters(
DocumentationHelper.tokenIdPathParameterDescriptor()
),
PayloadDocumentation.requestFields(
DocumentationHelper.keyAttributesFieldDescriptor()
),
PayloadDocumentation.relaxedResponseFields(
PayloadDocumentation.beneathPath("[]"),
DocumentationHelper.keyTypeFieldDescriptor(),
DocumentationHelper.keyIdFieldDescriptor(),
DocumentationHelper.keySizeFieldDescriptor("size"),
DocumentationHelper.curveOIDFieldDescriptor().type(String.class).optional(),
DocumentationHelper.publicKeyFieldDescriptor(),
DocumentationHelper.privateKeyFieldDescriptor()
),
PayloadDocumentation.responseFields(
PayloadDocumentation.beneathPath("[].privateKey"),
DocumentationHelper.keyUsagesFieldDescriptor(),
DocumentationHelper.keyAttributesFieldDescriptor()
),
PayloadDocumentation.responseFields(
PayloadDocumentation.beneathPath("[].publicKey"),
DocumentationHelper.rsaPublicKeyDetailsFieldDescriptors()
)
)
```
The `PayloadDocumentation.beneathPath("[]")` is not used at all, which result in multiple errors with fields not found.
We have seen in the documentation that it would be possible to use `subsectionWithPath` or `fieldWithPath` but since `MockMvcRestDocumentationWrapper` is described to be a drop-in replacement, we do not want to go this route.