Remove language about an experiment

Some of the language in this RFC implied that this is an experiment
and that another RFC would be required.  This isn't how we want to
frame things.  Another RFC could always be required, but it's also
possible that stabilization could occur on the basis of this RFC.

This is a normal RFC.  Let's remove all language about this being an
experiment.

text/0000-gen-fn.md

@@ -6,9 +6,7 @@
 # Summary
 [summary]: #summary
 
-Reserve the `gen` keyword in the 2024 edition and start experimenting on
-implementing generator functions and blocks that allow implementing `Iterator`s
-without writing `Iterator::next` impls under placeholder syntax.
+Reserve the `gen` keyword in the 2024 edition.
 
 Add `gen {}` blocks to the language. These implement `Iterator` by `yield`ing
 elements. This is simpler and more intuitive than creating a custom type and
@@ -23,11 +21,12 @@ Furthermore, add `gen fn` to the language. `gen fn foo(arg: X) -> Y` desugars to
 # Motivation
 [motivation]: #motivation
 
-The main motivation of this RFC is to reserve a new keyword in the 2024 edition.
-The feature used by the keyword described here should be treated as an e-RFC for
-experimentation on nightly. I would like to avoid discussion of the semantics
-provided here, deferring that discussion until during the experimental
-implementation work.
+The main motivation of this RFC is to reserve a new keyword in the 2024
+edition. We will discuss the semantic questions of generators in this
+document, but we do not have to settle them with this RFC. We'll describe
+current thinking on the semantics, but some questions will be left open to be
+answered at a later time after we gain more experience with the
+implementation.
 
 Writing iterators manually can be very painful. Many iterators can be written by
 chaining `Iterator` methods, but some need to be written as a `struct` and have
@@ -280,10 +279,8 @@ gen fn foo() {}
 
 ## Different syntax for `gen fn`:
 
-This RFC explicitly picks an attribute, as that has no conflicts with any other syntax, even within macros, and
-does not pick any option that may influence how experimental users think about syntax.
-There are many options to choose from, and we'll have to decide on one before stabilization.
-Some options are:
+This RFC selects `gen` as the keyword.  But there are other options we might
+pick.  Here are some alternatives:
 
 ```rust
 fn foo(args) yield item