Cleanup: move public doc-strings into headers for 'functions'

Ref T92709

source/blender/functions/FN_field.hh

@@ -405,6 +405,24 @@ class FieldEvaluator : NonMovable, NonCopyable {
   IndexMask get_evaluated_as_mask(const int field_index);
 };
 
+/**
+ * Evaluate fields in the given context. If possible, multiple fields should be evaluated together,
+ * because that can be more efficient when they share common sub-fields.
+ *
+ * \param scope: The resource scope that owns data that makes up the output virtual arrays. Make
+ *   sure the scope is not destructed when the output virtual arrays are still used.
+ * \param fields_to_evaluate: The fields that should be evaluated together.
+ * \param mask: Determines which indices are computed. The mask may be referenced by the returned
+ *   virtual arrays. So the underlying indices (if applicable) should live longer then #scope.
+ * \param context: The context that the field is evaluated in. Used to retrieve data from each
+ *   #FieldInput in the field network.
+ * \param dst_varrays: If provided, the computed data will be written into those virtual arrays
+ *   instead of into newly created ones. That allows making the computed data live longer than
+ *   #scope and is more efficient when the data will be written into those virtual arrays
+ *   later anyway.
+ * \return The computed virtual arrays for each provided field. If #dst_varrays is passed, the
+ *   provided virtual arrays are returned.
+ */
 Vector<GVArray> evaluate_fields(ResourceScope &scope,
                                 Span<GFieldRef> fields_to_evaluate,
                                 IndexMask mask,
@@ -434,6 +452,15 @@ template<typename T> Field<T> make_constant_field(T value)
 
 GField make_constant_field(const CPPType &type, const void *value);
 
+/**
+ * If the field depends on some input, the same field is returned.
+ * Otherwise the field is evaluated and a new field is created that just computes this constant.
+ *
+ * Making the field constant has two benefits:
+ * - The field-tree becomes a single node, which is more efficient when the field is evaluated many
+ *   times.
+ * - Memory of the input fields may be freed.
+ */
 GField make_field_constant_if_possible(GField field);
 
 class IndexFieldInput final : public FieldInput {