cli: clarify --debug flag's help

doc/lib.m4

@@ -70,8 +70,8 @@ m4_define({{_generaloptions_}}, {{
 `--version`
 : show version
 
-`--debug=N`
-: show debug output if N is 1-9 (default: 0)
+`--debug[=N]`
+: show debug output (levels 1-9, default: 1)
 
 `-f FILE --file=FILE`
 : use a different input file. For stdin, use -

hledger/Hledger/Cli/CliOptions.hs

@@ -103,7 +103,7 @@ helpflags = [
  ,flagNone ["man"]  (setboolopt "man")  "show the current program's manual with man"
  ,flagNone ["info"] (setboolopt "info") "show the current program's manual with info"
  -- ,flagNone ["browse-args"] (setboolopt "browse-args") "use a web UI to select options and build up a command line"
- ,flagReq  ["debug"]    (\s opts -> Right $ setopt "debug" s opts) "N" "show increasing amounts of debug output if N is 1-9. With no argument, show level 1"
+ ,flagReq  ["debug"]    (\s opts -> Right $ setopt "debug" s opts) "[N]" "show debug output (levels 1-9, default: 1)"
  ,flagNone ["version"] (setboolopt "version") "show version information"
  ]
 

hledger/doc/options.m4.md

@@ -53,7 +53,7 @@ If in doubt, keep things simple:
 - if needed, also add a backslash to escape regexp metacharacters
 - run add-on executables directly
 
-If you're really curious, add `--debug 2` for troubleshooting.
+If you're really curious, add `--debug=2` for troubleshooting.
 
 
 **General options:**