Update doc in parameter file

doc/RTD/source/GettingStarted/parameter_file.rst

@@ -12,8 +12,9 @@ Each section in this file corresponds to a different option in SWIFT and are not
 Output Selection
 ~~~~~~~~~~~~~~~~
 
-With SWIFT, you can select the output in the parameter file.
+With SWIFT, you can select the particle fields to output in snapshot using the parameter file.
 In section `SelectOutput`, you can remove a field by adding a parameter formatted in the following way `field_parttype` where `field` is the name of the field that you want to remove (e.g. `Masses`) and `parttype` is the type of particles that contains this field (e.g. `Gas`, `DM` or `Star`).
 For a parameter, the only values accepted are 0 (skip this field when writing) or 1 (default, do not skip this field when writing).
 
 You can generate a `yaml` file containing all the possible fields with `./swift -o output.yml`.
+By default, all the fields are written.