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.

An angular brace indicates an optional part; square brackets should be used to denote an argument