message: docs(api): update magdeck engage() doc
diff: @@ -176,13 +176,14 @@ Magnetic Module
  The magnetic module has two actions:
  -- engage: The magnetic stage rises to a default height unless otherwise specified
  +- engage: The magnetic stage rises to a default height unless an *offset* or a custom *height* is specified
  - disengage: The magnetic stage moves down to its home position
  -The magnetic module is currently compatible with normal PCR well plates. The magnets will default
  -to an engaged height of about 4.3 mm from the bottom of the well. This is roughly 30% of the well depth.
  -This engaged height has been tested for an elution volume of 40ul.
  +The magnetic module api is currently fully compatible with the BioRad Hardshell 96-PCR (.2ml) well plates. The magnets will
  +default to an engaged height of about 4.3 mm from the bottom of the well (or 18mm from magdeck home position). This is
  +roughly 30% of the well depth. This engaged height has been tested for an elution volume of 40ul.
  +You can also specify a custom engage height for the magnets so you can use a different labware with the magdeck.
  In the future, we will have adapters to support tuberacks as well as deep well plates.
  @@ -193,20 +194,38 @@ Engage
  from opentrons import modules, labware
  module = modules.load('magdeck', slot)
  - plate = labware.load('96-flat', slot, share=True)
  + plate = labware.load('biorad-hardshell-96-PCR', slot, share=True)
  module.engage()
  If you deem that the default engage height is not ideal for your applications,
  you can include an offset in mm for the magnet to move to. The engage function
  -will take in a value (positive or negative) to move the magnets from the **current**
  -offset position.
  +will take in a value (positive or negative) to offset the magnets from the **default** position.
  -To move the magnets higher than the default position you would input a positive mm value such as:
  -``module.engage(4)``
  +To move the magnets higher than the default position you would specify a positive mm offset such as:
  +``module.engage(offset=4)``
  To move the magnets lower than the default position you would input a negative mm value such as:
  -``module.engage(-4)``
  +``module.engage(offset=-4)``
  +
  +You can also use a custom height parameter with engage():
  +
  +.. code-block:: python
  +
  + from opentrons import modules, labware
  +
  + module = modules.load('magdeck', slot)
  + plate = labware.load('96-deep-well', slot, share=True)
  +
  + module.engage(height=12)
  +
  +The height should be specified in mm from the magdeck home position (i.e. the position of magnets when power-cycled or
  +disengaged)
  +
  +** Note **
  +`engage()` and `engage(offset=y)` can only be used for labware that have default heights defined in the api. If your
  +labware doesn't yet have a default height definition and your protocol uses either of those methods then you will get
  +an error. Simply use the height parameter to provide a custom height for you labware in such a case.
  Disengage
  =========
  @@ -215,7 +234,7 @@ Disengage
  from opentrons import modules, labware
  module = modules.load('magdeck', slot)
  - plate = labware.load('96-flat', slot, share=True)
  + plate = labware.load('biorad-hardshell-96-PCR', slot, share=True)
  module.engage()
  ## OTHER PROTOCOL ACTIONS
  