[DOXIA-762] Clarify code highlighting for verbatim text in Markdown and APT

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[DOXIA-762](https://issues.apache.org/jira/browse/DOXIA-762?redirect=false)** and commented

Currently it is undocumented how to emit `Sink.verbatim(SinkEventAttributes)` blocks with/without the `SinkEventAttributeSet.SOURCE` argument.

- For APT it is verbatim with source if using delimiter `+--`, verbatim without if using delimiter `---`. However in https://maven.apache.org/doxia/references/apt-format.html#verbatim-text only boxed and unboxed verbatim is described.
- For MD it is verbatim with source if fenced code with info string, e.g. ````java` and verbatim without source if fenced code without info string (`````). That is currently not mentioned at all in https://maven.apache.org/doxia/modules/index.html#Markdown nor in https://maven.apache.org/doxia/doxia/doxia-modules/doxia-module-markdown/index.html.

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**Issue Links:**
- [DOXIA-685](https://issues.apache.org/jira/browse/DOXIA-685) Replace SinkEventAttributes#BOXED with #SOURCE to clearly separate between regular verbatim and verbatim source (code)

- [DOXIA-763](https://issues.apache.org/jira/browse/DOXIA-763) MarkdownSink: Distinguish between verbatim with source attribute and without