add granular exit codes to the enhacements of easybuild v5

docs/easybuild-v5/enhancements.md

@@ -5,10 +5,23 @@
 Various significant enhancements are included in EasyBuild v5.0, including:
 
 * [`run_shell_cmd` function][run_shell_cmd]
+* [Granular exit codes][granular_exit_codes]
 
 ---
 
 ## `run_shell_cmd` function { : #run_shell_cmd }
 
 See dedicated page on the new [`run_shell_cmd` function](run_shell_cmd.md).
 
+## Granular exit codes { : #granular_exit_codes }
+
+EasyBuild v5 now uses a range of ~50 exit codes instead of just 0 for normal
+termination and 1 for unexpected termination. Each non-zero exit code
+correlates to the specific type of error or failure that caused the
+termination of the program. For instance, a missing easyconfig or a failed
+checksum check. The full list of exit codes is defined in the class
+[easybuild.tools.build_log.EasyBuildExit](https://github.com/easybuilders/easybuild-framework/blob/main/easybuild/tools/build_log.py#L74).
+
+EasyBuild will always return its own exit codes on termination. Other exit
+codes from external processes executed through `run_shell_cmd` or HTTP response
+status codes are reported in the corresponding logs.