[REQ] Clarify heading level for rendered inserts

[mhatzl]

Other Unimarkup files may be inserted using rendered file inserts.
Some markup languages like AsciiDoc allow to specify that all inserted headings start at a higher level.
However, this makes file insertion more difficult, and also a bit confusing, because in the inserted file, heading level 6 may be used, which makes it impossible to add the start-offset.

Therefore, Unimarkup should not allow this feature.
To make this behaviour clear, clarification should be added to the rendered file insert section.