Refactor return code docs [1.4-maint]

docs/usage/general/return-codes.rst.inc

@@ -7,16 +7,37 @@ Borg can exit with the following return codes (rc):
 Return code Meaning
 =========== =======
 0           success (logged as INFO)
-1           generic warning (operation reached its normal end, but there were warnings --
-            you should check the log, logged as WARNING)
-2           generic error (like a fatal error, a local or remote exception, the operation
-            did not reach its normal end, logged as ERROR)
-3..99       specific error (enabled by BORG_EXIT_CODES=modern)
-100..127    specific warning (enabled by BORG_EXIT_CODES=modern)
-128+N       killed by signal N (e.g. 137 == kill -9)
+1           generic warning (operation reached its normal end, but there
+            were warnings - you should check the log; logged as WARNING)
+2           generic error (such as a fatal error or a local/remote exception;
+            the operation did not reach its normal end; logged as ERROR)
+3..99       specific error (enabled by ``BORG_EXIT_CODES=modern``,
+            see below; logged as ERROR)
+100..127    specific warning (enabled by ``BORG_EXIT_CODES=modern``,
+            see below; logged as WARNING)
+128+N       terminated by signal N (e.g. 130 == SIGINT, Ctrl+C, or kill -2;
+            logged as ERROR)
 =========== =======
 
 If you use ``--show-rc``, the return code is also logged at the indicated
 level as the last log entry.
 
-The modern exit codes (return codes, "rc") are documented there: :ref:`msgid`
+Borg categorizes return codes into groups and exits with the more severe
+group: signals (rc 128+N) are more severe than errors (rc 2 and 3..99),
+which take precedence over warnings (rc 1 and 100..127), and lastly
+success (rc 0).
+
+Borg 1 exits with the generic error (rc 2) and warning (rc 1) codes
+by default. If you want Borg 1 to exit with specific error (rc 3..99)
+and warning (rc 100..127) codes instead (like Borg 2 does), set the
+``BORG_EXIT_CODES=modern`` environment variable.
+
+If modern exit codes are enabled (``BORG_EXIT_CODES=modern``):
+Within the signal and error groups, the first signal or error determines
+the final return code. Within the warning group, Borg returns the specific
+warning code (rc 100..127) if there were one or more warnings of the same
+kind. If warnings of different kinds occurred, Borg returns the generic
+warning code (rc 1) instead. All errors and warnings are still logged
+individually.
+
+For a list of all specific error and warning codes, see :ref:`msgid`.