make the introduction of the command-by-category list less confusing

doc/src/Section_commands.txt

@@ -281,12 +281,12 @@ the "minimize"_minimize.html command.  A parallel tempering
 
 3.4 Commands listed by category :link(cmd_4),h4
 
-This section lists all LAMMPS commands, grouped by category.  The
-"next section"_#cmd_5 lists the same commands alphabetically.  The
+This section lists core LAMMPS commands, grouped by category.
+The "next section"_#cmd_5 lists all commands alphabetically.  The
 next section also includes (long) lists of style options for entries
 that appear in the following categories as a single command (fix,
 compute, pair, etc).  Commands that are added by user packages are not
-included in these categories, but they are in the next section.
+included in the categories here, but they are in the next section.
 
 Initialization: