docs: porting more README and save method

docs/dsl/png.rst

@@ -8,7 +8,7 @@ Options
 .. include:: /args/expansion.rst
 
 file
-  default: ``''``
+  default: ``''`` (empty string)
 
   file(s) to read in. As in :doc:`/arrays`, if this a single file, then it's use for every card in range. If the parameter is an Array of files, then each file is looked up for each card. If any of them are nil or '', nothing is done for that card.
 

docs/dsl/save.rst

@@ -1,2 +1,23 @@
 save
-----
+====
+
+Saves the given range of cards to either PNG or PDF. Wrapper method for other save methods.
+
+Options
+-------
+
+This method delegates everything to :doc:`save_png` or :doc:`save_pdf` using the ``format`` option. All other options are passed along.
+
+format
+  default: ``[]`` (do nothing)
+
+  Use ``:png`` to save as a PNG, and ``:pdf`` to save as PDF. To save to both at once, use ``[:png, :pdf]``
+
+Examples
+--------
+
+::
+
+  save format: :png, prefix: 'front_' # same as: save_png prefix: 'front_'
+  save format: :pdf, prefix: 'cards_' # same as: save_pdf prefix: 'cards_'
+  save format: [:png, :pdf]           # same as: save_png; save_pdf