[Docs] Clarify accessing Date methods in painless (elastic#33560)

The documentation currently tells users to use `doc['event_date'].value.getMillis` to access
milliseconds in a date. It turns out the way it works is `doc['event_date'].value.millis`. This 
change corrects this and gives a hint at how other date related methods work.

docs/painless/painless-getting-started.asciidoc

@@ -198,13 +198,11 @@ POST hockey/player/1/_update
 ==== Dates
 
 Date fields are exposed as
-`ReadableDateTime` or
-so they support methods like
-`getYear`,
-and `getDayOfWeek`.
-To get milliseconds since epoch call
-`getMillis`.
-For example, the following returns every hockey player's birth year:
+`ReadableDateTime`, so they support methods like `getYear`, `getDayOfWeek` 
+or e.g. getting milliseconds since epoch with `getMillis`. To use these
+in a script, leave out the `get` prefix and continue with lowercasing the
+rest of the method name. For example, the following returns every hockey 
+player's birth year:
 
 [source,js]
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