some error hint updates based on recent discussions on MatSci.org

doc/src/Errors_details.rst

@@ -51,8 +51,11 @@ Parallel versus serial
 ^^^^^^^^^^^^^^^^^^^^^^
 
 Issues where something is "lost" or "missing" often exhibit that issue
-only when running in parallel.  That doesn't mean there is no problem,
+*only* when running in parallel.  That doesn't mean there is no problem
-only the symptoms are not triggering an error quickly.  Correspondingly,
+when running in serial, only the symptoms are not triggering an error.
+This may be because there is no domain decomposition with just one
+processor and thus all atoms are accessible, or it may be because the
+problem will manifest faster with smaller subdomains.  Correspondingly,
 errors may be triggered faster with more processors and thus smaller
 sub-domains.
 
@@ -244,6 +247,25 @@ equal style (or similar) variables can only be expanded before the box
 is defined if they do not reference anything that cannot be defined
 before the box (e.g. a compute or fix reference or a thermo keyword).
 
+.. _hint13:
+
+Illegal ... command
+^^^^^^^^^^^^^^^^^^^
+
+These are a catchall error messages that used to be used a lot in LAMMPS
+(also programmers are sometimes lazy).  They usually include the name of
+the source file and the line where the error happened.  This can be used
+to track down what caused the error (most often some form of syntax error)
+by looking at the source code.  However, this has two disadvantages: 1. one
+has to check the source file from the exact same LAMMPS version, or else
+the line number would be different or the core may have been rewritten and
+that specific error does not exist anymore.
+
+The LAMMPS developers are committed to replace these too generic error
+messages with more descriptive errors, e.g. listing *which* keyword was
+causing the error, so that it will be much simpler to look up the
+correct syntax in the manual (and without referring to the source code).
+
 ------
 
 .. _err0001: