Add dump adios to PDF build. Update example config with better documentation

doc/src/dump_adios.txt

@@ -55,7 +55,7 @@ write_dump all atom/adios singledump.bp :pre
 
 The number of atoms per snapshot CAN change with the adios style.
 When using the ADIOS tool 'bpls' to list the content of a .bp file,
-bpls will print "__" for the size of the output table indicating that
+bpls will print {__} for the size of the output table indicating that
 its size is changing every step. 
 
 The {atom/adios} and {custom/adios} dump styles are part of the USER-adios package. 

doc/src/lammps.book

@@ -150,6 +150,7 @@ dielectric.html
 dimension.html
 displace_atoms.html
 dump.html
+dump_adios.html
 dump_h5md.html
 dump_image.html
 dump_modify.html

examples/USER/adios/adios2_config.xml

@@ -6,8 +6,15 @@
         <engine type="BPFile">
             <parameter key="substreams" value="10"/>
         </engine>
+
         <engine type="HDF5">
 
+
+        The 'substreams' parameter in BPFile controls how many 
+        files on disk are created. This number should be proportional 
+        to the number of servers in the parallel file system, 
+        NOT to the number of processes. 
+        substreams=1 is generally a very inefficient setting for large parallel runs.
     -->
 
     <!--====================================================
@@ -16,6 +23,7 @@
 
     <io name="atom">
         <engine type="BPFile">
+            <parameter key="substreams" value="1"/>
         </engine>
     </io>
 
@@ -25,6 +33,7 @@
 
     <io name="custom">
         <engine type="BPFile">
+            <parameter key="substreams" value="1"/>
         </engine>
     </io>