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

[mr-c]

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

[mr-c]

@tetron pointed out that this is already allowed:

http://www.commonwl.org/v1.0/CommandLineTool.html#Parameter_references "Multiple parameter references may appear in a single field. This case must be treated as a string interpolation. After interpolating the first parameter reference, interpolation must be recursively applied to the trailing characters to yield the final string value."

Tested at https://github.com/common-workflow-language/common-workflow-language/blob/master/v1.0/v1.0/params_inc.yml#L72