fix Issue 9713 - Ddoc: Empty description suppress automatic example generation

Even if proper description is empty, generated examples should be appeared
fix Issue 9714 - Ddoc: Combination of -D and -unittest reveals hidden unittest function

Even if -unittest is specified, hidden function should not be appeared in ddoc output
Refinement for #1641

- Reuse unindent mechanism that has introduced by #1377
- Remove trailing whitespace from auto-generated ddoc example
fix Issue 9757 - Ddoc: documented unittest after ditto should work
Merge pull request #1746 from 9rnsr/fix_ddoc

Issue 9713 & 9714 - Ddoc bug fixes