add -nonbuf / -nb command line flag to disable buffering for screen and logfile

doc/src/Run_options.rst

@@ -14,6 +14,7 @@ letter abbreviation can be used:
 * :ref:`-m or -mpicolor <mpicolor>`
 * :ref:`-c or -cite <cite>`
 * :ref:`-nc or -nocite <nocite>`
+* :ref:`-nb or -nonbuf <nonbuf>`
 * :ref:`-pk or -package <package>`
 * :ref:`-p or -partition <partition>`
 * :ref:`-pl or -plog <plog>`
@@ -257,6 +258,23 @@ Disable generating a citation reminder (see above) at all.
 
 ----------
 
+.. _nonbuf:
+
+**-nonbuf**
+
+Turn off buffering for screen and logfile output.  For performance
+reasons, output to the screen and logfile is usually buffered, i.e.
+output is only generated if a buffer - typically 4096 bytes - has been
+filled.  However, when LAMMPS crashes, that can mean that there is
+important output missing.  When using this flag, this buffering is
+turned off (only for screen and logfile output).  Note that when running
+in parallel with MPI, the screen output may still be buffered by the MPI
+library which cannot be changed by LAMMPS.  This flag should only be
+used for debugging and not for production simulations as the performance
+impact can be significant, especially for large parallel runs.
+
+----------
+
 .. _package:
 
 **-package style args ....**