document in user guide that multiple parameter references and string concatenation are possible without InlineJavascript

Inspired by https://www.biostars.org/p/234804/#238901

Implementers, does the following seem reasonable?
Note the lack of `InlineJavascriptRequirement`
```
cwlVersion: v1.0
class: Workflow
requirements:
ScatterFeatureRequirement: {}
StepInputExpressionRequirement: {}
inputs:
bam: File
intervals: string[]
outputs: []
steps:
hc:
run: dummy_HC.cwl
scatter: [interval_string]
in:
inputBam: bam
interval_string: intervals
outputVcf:
valueFrom: $(inputs.inputBam.nameroot).$(inputs.interval_string).vcf
```