Update separate-compilation.md

added sentence about translation units

Author: VictorEijkhout

sources/modules/compilation-model/separate-compilation.md

@@ -78,10 +78,11 @@ implementation-defined, unspecified, or undefined behavior._
 
 A student should be able to:
 
-1. Use compile flags, including `-I` and `-L` to specify search paths.
-2. Make header files for their own code, including using header guards.
-3. Use include directives for the headers of external libraries.
-4. Be able to declare `extern` variables.
+1. Explain what a translation unit is, and its relation to header files.
+2. Use compile flags, including `-I` and `-L` to specify search paths.
+3. Make header files for their own code, including using header guards.
+4. Use include directives for the headers of external libraries.
+5. Be able to declare `extern` variables.
 
 #### Caveats