Update PSF reffile docs

Author: melanieclarke

docs/jwst/references_general/psf_reffile.inc

@@ -20,14 +20,16 @@ All keywords used for file selection are *required*.
 Instrument Keywords
 ========== =========================================================================
 MIRI       INSTRUME, DETECTOR, FILTER, EXP_TYPE
+NIRSpec    INSTRUME, DETECTOR, FILTER, EXP_TYPE
 ========== =========================================================================
 
 .. include:: ../includes/standard_keywords.inc
 
 Reference File Format
 +++++++++++++++++++++
-PSF reference files are in FITS format, with 2 IMAGE extensions.
-The FITS primary HDU does not contain a data array.
+PSF reference files are in FITS format, with 2 IMAGE extensions for each
+included aperture. The FITS primary HDU does not contain a data array.
+
 The format and content of the file is as follows:
 
 =======  ========  =====  ==============  =========
@@ -42,3 +44,8 @@ position, at each dispersion element specified in the ``WAVE`` array.  Detector
 pixels are subsampled by the amount specified in the SUBPIX keyword, and the PSF
 is centered on the cross-dispersion element specified in either CENTCOL (for vertical
 dispersion) or CENTROW (for horizontal dispersion).
+
+For MIRI LRS data, the PSF reference file contains only one each of the
+``PSF`` and ``WAVE`` arrays.  For NIRSpec fixed slit, it contains 5 each of
+these arrays, one for each supported slit. The slit name is recorded in the
+APERTURE keyword in the PSF extension.