Clarify differences between value and expression syntax

[l0b0]

Passing values with a path containing a hyphen is different from how they have to be referenced in expressions:

$ argo-expr "{{=inputs.parameters['may-contain-quotes'] == 'example'}}" --value "inputs.parameters['may-contain-quotes']=example"
failed to evaluate expression: cannot fetch may-contain-quotes from <nil> (1:18)
 | inputs.parameters['may-contain-quotes'] == 'example'
 | .................^
$ argo-expr "{{=inputs.parameters['may-contain-quotes'] == 'example'}}" --value inputs.parameters.may-contain-quotes='example'
true