Improve documentation of simplified [] and ? types for advanced use cases

[fmigneault]

In https://www.commonwl.org/v1.2/CommandLineTool.html#Document_preprocessing

2.4 Document preprocessing

Another transformation defined in Schema salad is simplification of data type definitions. Type ending with ? should be transformed to [, "null"]. Type ending with [] should be transformed to {"type": "array", "items": }

There is no explicit mention of combining the two simplified type definitions.
This leaves room for interpretation.

For example: string[]?
Should this be understood as:

1. required array of optional strings, ie:

type: array
items: [string, 'null']

2. optional array of required strings. ie:

type: 
  - type: 'null'
  - type: array
    items: string

3. both are optional, ie:

type: 
  - type: 'null'
  - type: array
    items: [string, 'null']

4. neither (ie: disallowed to combine these shortcuts - I don't think so, because I've used them, but docs should mention it)
5. Can the reverse be applied (ie: string?[])
   • Does it have the same meaning?
   • Is the order of ?/[] relevant?