Flux function regexp.replaceAllString should describe how expansion works

The documentation for the Flux function `regexp.replaceAllString` should describe that one can capture text from the regexp and then reuse it to replace text with `$1`, etc, as in this example:
https://golang.org/pkg/regexp/#Regexp.ReplaceAll

In general, Flux regexp functions are backed by equivalent functions in the Go standard library.

##### Relevant URLs
https://docs.influxdata.com/influxdb/v2.0/reference/flux/stdlib/regexp/replaceallstring/

##### What products and version are you using?

2.0 Cloud