Clarify difference between `metadata.licenses` and `metadata.component.licenses`

[lfrancke]

The JSON Schema currently has this snippet for metadata.licenses:

        "licenses": {
          "title": "BOM License(s)",
          "$ref": "#/definitions/licenseChoice"
        },

This gets rendered as:

The title is not shown making it unclear what the difference is between those two.

I suggest adding a description.

Similar for XML:

            <xs:element name="licenses" type="bom:licenseChoiceType" minOccurs="0" maxOccurs="1"/>

[jkowalleck]

metadata.licenses is the license of the CycloneDX document.
the other one is the license of the component descibed by the document.

I will update the documentation on the schema files.

[jkowalleck]

could I ask you for a review of #378 ?