Fix file name and extend keyword description (dftbplus#1446)

* Fix file name and extend keyword description

* Apply suggestions from code review

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Co-authored-by: Ben Hourahine <[email protected]>

Author: aradi

doc/dftb+/manual/dftbp.tex

@@ -5867,11 +5867,14 @@ \section{ElectronDynamics}
 \item[\is{WriteAtomicEnergies}] If the atom-resolved contributions to the total energy
 shpuld be written to the \verb|atomenergies.dat| file.
 
-\item[\is{FillingsFromFile}] If the initial fillings (molecular
-  orbital occupations) should be read from a file named
-  ``fillings.ini'' present in the working directory. If used, the
-  ground state fillings are replaced by these and used to build the
-  initial density matrix.
+\item[\is{FillingsFromFile}] Whether to read the initial molecular orbital
+    occupations from a file. When set to \is{Yes}, please place a file named
+    ``fillings.in'' in the working directory. This file will override the
+    default ground state fillings when constructing the initial density matrix.
+    The file should list the occupation for each orbital, one per line, for each spin
+    channel (starting with all of the orbitals for the first spin channel). For
+    spin-polarized calculations with collinear spin, the occupations for the
+    second spin channel should follow sequentially, while for non-collinear/spin-orbit calculations all orbitals should be listed in ascending energy.
 
 \end{description}