Document `fieldpath` semantics

### What happened?

Setting an annotation or label via `SetValue` using a label or annotation key with a "." is set incorrectly ie: `prometheus.io.metrics`

It will be set as

```yaml
metadata:
labels:
prometheus:
io:
metrics: /metrics
```

It should result in

```yaml
metadata:
labels:
prometheus.io.metrics: /metrics
```

This seems to be more of an issue with the `SetValue` in `fieldpath` Pave objects.

I understand that you can use `SetAnnotations` or `SetLabels` but this requires either merging the existing annotations together, which would often take longer than setting the fields individually, or requires knowing the full annotations or labels at the start. either way it would be nice to use SetValue this way as well. If it is not intended to be used this way, perhaps a comment suggesting alternatives would be good

### How can we reproduce it?

using this `composed_test` file

```go
package composed

import (
"fmt"
"testing"

"sigs.k8s.io/yaml"
)

func TestSetValue(t *testing.T) {
cd := New()
cd.SetAPIVersion("example.org/v1")
cd.SetKind("CoolComposedResource")

cd.SetValue("metadata.labels.prometheus.io.metrics", "/metrics")
cd.SetValue("metadata.labels.\"prometheus.io.url\"", "https://exampleprom.io")
cd.SetValue("metadata.annotations.prometheus.io.metrics", "/metrics")
cd.SetValue("metadata.annotations.\"prometheus.io.url\"", "https://exampleprom.io")

y, _ := yaml.Marshal(cd)

fmt.Println(string(y))
}
```

you will see the following output

```yaml
$ go test -v ./...
=== RUN TestSetValue
apiVersion: example.org/v1
kind: CoolComposedResource
metadata:
annotations:
prometheus:
io:
metrics: /metrics
url: https://exampleprom.io
labels:
prometheus:
io:
metrics: /metrics
url: https://exampleprom.io

--- PASS: TestSetValue (0.00s)
PASS
ok github.com/crossplane/function-sdk-go/resource/composed 0.226s
```

### What environment did it happen in?
Crossplane version: Latest version