[BUGFIX] Improve description of registerModule()

The description is wrong about how a new main module
is created.

Change-Id: I5bf0b793258a81b8cc89d1e9e653a9f63bbeb4f6
Resolves: #10250
Releases: 6.1
Reviewed-on: https://review.typo3.org/19965
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn