document the --incremental build switch; make CIs green again

compiler/commands.nim

@@ -642,7 +642,8 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
   of "help", "h":
     expectNoArg(conf, switch, arg, pass, info)
     helpOnError(conf, pass)
-  of "symbolfiles", "incremental":
+  of "symbolfiles": discard "ignore for backwards compat"
+  of "incremental":
     when not defined(nimIncremental):
       localError(conf, info, "the compiler was not built with " &
         "incremental compilation features; bootstrap with " &
@@ -653,7 +654,7 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     of "writeonly": conf.symbolFiles = writeOnlySf
     of "readonly": conf.symbolFiles = readOnlySf
     of "v2": conf.symbolFiles = v2Sf
-    else: localError(conf, info, "invalid option for --symbolFiles: " & arg)
+    else: localError(conf, info, "invalid option for --incremental: " & arg)
   of "skipcfg":
     expectNoArg(conf, switch, arg, pass, info)
     incl(conf.globalOptions, optSkipSystemConfigFile)

doc/advopt.txt

@@ -101,6 +101,7 @@ Advanced options:
   --listCmd                 list the commands used to execute external programs
   --parallelBuild:0|1|...   perform a parallel build
                             value = number of processors (0 for auto-detect)
+  --incremental:on|off      only recompile the changed modules (experimental!)
   --verbosity:0|1|2|3       set Nim's verbosity level (1 is default)
   --experimental:$1
                             enable experimental language feature