Update imprint documentation to mention association input

docs/jwst/imprint/description.rst

@@ -7,14 +7,18 @@ Description
 The NIRSpec MSA imprint subtraction step removes patterns created in NIRSpec
 MOS and IFU exposures by the MSA structure. This is accomplished by
 subtracting a dedicated exposure taken with all MSA shutters closed and the
-IFU entrance aperture blocked.
+IFU entrance aperture blocked.  These exposures are called "imprint" or "leakcal"
+images.
 
-The step has two input parameters: the target exposure and a list of one
-or more imprint exposures. These arguments can be provided as either file names
-or JWST data models.
+The imprint subtraction step has two input parameters: the target exposure and a
+list of one or more imprint exposures. When called as a standalone step, these arguments
+can be provided as either file names or JWST data models.  When the step is called in
+the context of the :ref:`calwebb_spec2 <calwebb_spec2>` pipeline, imprint file names
+should be specified in the input `spec2` association file, labeled with the member type
+of "imprint".
 
 In the event that multiple imprint images are provided, the step uses the
-meta data of the target and imprint exposures to find the imprint exposure
+metadata of the target and imprint exposures to find the imprint exposure
 that matches the observation number (keyword "OBSERVTN"), dither pattern
 position number (keyword "PATT_NUM"), and background target flag
 (keyword "BKGDTARG") of the input exposure. The matching
@@ -25,7 +29,7 @@ unaltered.
 When subtracting the imprint data model from the target data model,
 the SCI data array of the imprint exposure is subtracted from the SCI array
 of the target exposure, and the DQ arrays of the two exposures are combined using
-a bitwise logical OR operation. The ERR and variance arrays are not
+a bitwise logical OR operation. The error and variance arrays are not
 currently used or modified.
 
 Step Arguments