Better doc molinfo

MOLTYPE is actually not needed in practice.

src/core/SetupMolInfo.cpp

@@ -43,7 +43,7 @@ void SetupMolInfo::registerKeywords( Keywords& keys ) {
   keys.add("compulsory","STRUCTURE","a file in pdb format containing a reference structure. "
            "This is used to defines the atoms in the various residues, chains, etc . "
            "For more details on the PDB file format visit http://www.wwpdb.org/docs.html");
-  keys.add("compulsory","MOLTYPE","protein","what kind of molecule is contained in the pdb file");
+  keys.add("compulsory","MOLTYPE","protein","what kind of molecule is contained in the pdb file - usually not needed since protein/RNA/DNA are compatible");
   keys.add("atoms","CHAIN","(for masochists ( mostly Davide Branduardi ) ) The atoms involved in each of the chains of interest in the structure.");
 }
 

src/setup/MolInfo.cpp

@@ -44,12 +44,13 @@ If you are using gromacs, the safest way is to use reference pdb file
 generated with `gmx editconf -f topol.tpr -o reference.pdb`.
 
 
-Using MOLINFO with a protein's pdb extend the possibility of atoms selection using the @ special
+Using MOLINFO with a protein's or nucleic acid's pdb extends the possibility of atoms selection using the @ special
 symbol.
 
 Providing `MOLTYPE=protein`, `MOLTYPE=rna`, or `MOLTYPE=dna` will instruct plumed to look
-for known residues from these three types of molecule (so that any of these three choice
-can be safely used in a RNA/protein complex).
+for known residues from these three types of molecule. In other words, this is available for
+historical reasons and to allow future extensions where alternative lists will be provided.
+As of now, you can just ignore this keyoword.
 
 For protein residues, the following groups are available: