man: improve exit code descriptions

man/exit-code.inc

@@ -1,8 +1,20 @@
 * EXIT CODE
 
 This command returns 0 upon successful completion, or a non-zero exit code
-otherwise.
+otherwise. Typical values are 2 (no matches found), 11 (database schema
+mismatch) and 12 (failed to acquire database lock).
 
+** no matches found (2)
+
+Nothing matching found; try a different query
+
+** database schema mismatch (11)
+
+You need to re-initialize ~mu~, see *mu-init(1*)
+
+** failed to acquire lock (19)
+
+Some other program has exclusive access to the mu (Xapian) database
 
 # Local Variables:
 # mode: org