Added module docs for selective execution

[lefou]

The hope is, to have some meaningfull mill inspect selective output, but it seems, inspect does not output the Scaladoc for modules yet.

Fixed some typos along the way.

[lihaoyi]

The scaladoc inspect thing should be fixed in #4576

[lihaoyi]

At least for some modules; I didn't test it with the lazy val approach to defining modules so may need more tweaks

[lefou]

At least for some modules; I didn't test it with the lazy val approach to defining modules so may need more tweaks

Looking into it.

It works nicely with object, but not with lazy val.

[62/66] ============================== inspect selective ============================== 60s
selective(MainModule.scala:327)

Inherited Modules:
    mill.main.SelectiveExecutionModule

diff --git a/main/src/mill/main/MainModule.scala b/main/src/mill/main/MainModule.scala
index 8e6974e331e..26149d1d76d 100644
--- a/main/src/mill/main/MainModule.scala
+++ b/main/src/mill/main/MainModule.scala
@@ -324,7 +324,8 @@ trait MainModule extends BaseModule {
    *   prints out the tasks in `<selector>` that are affected by the code changes
    *   and would have run, without actually running them.
    */
-  lazy val selective: SelectiveExecutionModule = new SelectiveExecutionModule {}
+  object selective extends SelectiveExecutionModule
 
 }

============================== inspect selective ==============================
[1/1] inspect
selective(MainModule.scala:328)
    Commands related to selective execution, where Mill runs tasks selectively
    depending on what task inputs or implementations changed.
    
    Read more about it at: https://mill-build.org/mill/large/selective-execution.html
    
    Here are the essential commands:
    
    - `mill selective.prepare <selector>`:
    run on the codebase before the code change,
    stores a snapshot of task inputs and implementations
    
    - `mill selective.run <selector>`:
    run on the codebase after the code change,
    runs tasks in the given `<selector>` which are affected by the code changes
    that have happened since `selective.prepare` was run
    
    - `mill selective.resolve <selector>`:
    a dry-run version of `selective.run`,
    prints out the tasks in `<selector>`` that are affected by the code changes
    and would have run, without actually running them.

Inherited Modules:
    mill.main.SelectiveExecutionModule