more diagrams

doc/images/Makefile.am

@@ -3,6 +3,7 @@
 # Ed Hartnett 5/25/19
 
 # These are the images used in the documentation.
-EXTRA_DIST = block-cyclic.png block-cyclic-rearr.png dof.png		\
-dof-rearr.png PIO_Intracomm1.png PIO_Library_Architecture1.jpg		\
-PIO_Decomposition.png I_O_on_Few.png I_O_on_Many.png PIO_Async.png
+EXTRA_DIST = block-cyclic.png block-cyclic-rearr.png dof.png	\
+dof-rearr.png PIO_Intracomm1.png PIO_Library_Architecture1.jpg	\
+PIO_Decomposition.png I_O_on_Few.png I_O_on_Many_Intracomm.png	\
+I_O_on_Many_Async.jpg PIO_Async.png

doc/source/Introduction.txt

@@ -15,12 +15,20 @@ of the processors will have to wait to do I/O.
 An obvious solution is to designate a small number of processors to do
 I/O, and use the rest for computation.
 
-@image html I_O_on_Many.png "I/O on Many Processors"
+@image html I_O_on_Many_Intracomm.png "I/O on Many Processors Intracomm"
 
 PIO provides a netCDF-like API which provides this service. User code
 is written as if parallel I/O is being used from every processor, but,
 under the hood, PIO uses the I/O processors to do all data access.
 
+With Intracomm Mode, the I/O processors are a subset of the
+computational processors, and only one computational unit is
+supported. In Async Mode, the I/O processors are dedicated to I/O, and
+do not perform computation. Also, more than one computational unit may
+be designated.
+
+@image html I_O_on_Many_Async.png "I/O on Many Processors Async"
+
 The user initializes the PIO IO System, designating some processors
 for I/O, others for computation.