Expand documentation of dynasave

The description of FILENAME is technically incorrect. It need not be quoted, if there is no extension. There is already a description of what is allowed in FILENAME (see the beginning of the-model-file.rst), so I'd rather remove the details added in this commit.

@sebastien That is a tricky issue, because the Matlab save-command in line 10441 will require a quoted string.

Indeed, thanks for pointing this. But there's nothing tricky: it’s just that the Dynare and MATLAB commands accept different syntaxes. That should normally be reflected in the documentation (i.e. the description of the MATLAB command should be modified in some way, since the FILENAME keyword is already reserved for the syntax accepted by Dynare commands).

Ok. Then we should adjust the documentation accordingly.