DDSSEC_ — Builtin Access Control - Use of 'TopicExpression' in Domain Governance

The Domain Governance schema defines a 'TopicExpression' as same as 'string' (simple inheritance). Why not use the 'string' type directly instead?
Also the description in 9.4.1.2.5.1 says it is a set of topic names but does not say how we specify such a set. This is better defined in later sections such as 9.4.1.3.2.3.1.2. I suggest to add the description of topic name expressions from 9.4.1.3.2.3.1.2 to 9.4.1.2.5.1, as it is more precise:

"The Topic name expression syntax and
matching shall use the syntax and rules of the POSIX fnmatch() function as specified in POSIX 1003.2-1992, Section B.6 [38]"

Add description of TopicExpression to 9.4.1.2.5.1

See issue description