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.

I do not understand why this is added. `@license` tags are commonly applied to every file because there is no authority representing a single package or namespace