[clang-tools-extra] [clang-doc] add tags to Mustache namespace template (PR #142045)

[Erick Velez via cfe-commits]

evelez7 wrote:

I cannibalized the tags from class-template.mustache. By happy accident, I realized that there is an `extractScopeChildren` so enums can be generated, which means the placeholders in `generateDocForInfo` (`OS << IT_enum`) can be filled in if the logic is moved to their own functions.

https://github.com/llvm/llvm-project/pull/142045