Make clear that passing model XML is valid on command line

man/man1/openmc.1

@@ -8,7 +8,10 @@ sections are available.
 .SH SYNOPSIS
 \fBopenmc\fR [\fIoptions\fR] [\fIpath\fR]
 .PP
-It is assumed that if no
+.I path
+specifies either the path to a single model XML file containing the full model
+or a directory containing either a model.xml file or a set of individual XML
+files (settings.xml, materials.xml, geometry.xml). It is assumed that if no
 .I path
 is specified, the XML input files are present in the current directory.
 .SH OPTIONS
@@ -46,6 +49,9 @@ The behavior of
 .B openmc
 is affected by the following environment variables.
 .TP
+.B OPENMC_CHAIN_FILE
+Indicates the path to a depletion chain XML file.
+.TP
 .B OPENMC_CROSS_SECTIONS
 Indicates the default path to the cross_sections.xml summary file that is used
 to locate HDF5 format cross section libraries if the user has not specified the

src/output.cpp

@@ -268,7 +268,7 @@ void print_usage()
 {
   if (mpi::master) {
     fmt::print(
-      "Usage: openmc [options] [directory]\n\n"
+      "Usage: openmc [options] [path]\n\n"
       "Options:\n"
       "  -c, --volume           Run in stochastic volume calculation mode\n"
       "  -g, --geometry-debug   Run with geometry debugging on\n"