Synposis + description of `@license` needs polishing

Objective:

1. Current syntax/synopsis suggests that a `@license` tag would work on its own (sans parameters), which is wrong.
1. SPDX identifiers should be recommended in favor of custom URLs. (standards)
1. Overall wording could use some polishing.

This sentence can better be split into two parts; the commas are not part of the same summary or sub-sentence