Fix documentation issues for interactive MD module.

This patch fixes three warnings reported by Teemu's doxygen documentation
checker (gerrit.gromacs.org/#/c/3213/13). I additionally added a
\libinternal to the module_imd in imd.h to suppress an (empty) IMD module
from occurring in the user documentation.

Change-Id: I01324ec17692419274a605745716f25321d5d729