Docs: document double translation better

Only document the use of `--enable effects` for now, as only this option
is correctly handled by Dune, and document possible caching issues with
`--effects=double-translation`.

manual/effects.wiki

@@ -1,7 +1,7 @@
 == Effect handlers ==
 
-Js_of_ocaml supports effect handlers with the {{{--effects={cps,double-translation}}}}
-flag. The {{cps}} option is based on partially transforming the program to
+Js_of_ocaml supports effect handlers with the {{{--enable=effects}}}
+flag. This is based on partially transforming the program to
 continuation-passing style.
 As a consequence, [[tailcall|tail calls]] are also fully optimized.
 This is not the default for now since the generated code can be slower,
@@ -11,7 +11,7 @@ The analysis is especially effective on monomorphic code. It is not so effective
 We hope to improve on this by trying alternative compilation
 strategies.
 
-The {{double-translation}} option does a similar CPS transform, but also keeps a direct-style version of the transformed functions. The choice of running the CPS version is delayed to run time. Since CPS code is usually slower, this can avoid degradations. In addition, one can ensure that some code is run in direct style by using {{Jsoo_runtime.Effect.assume_no_perform}}.
+An alternative CPS transform is provided under the {{--effects=double-translation}} option. It keeps a direct-style version of the transformed functions in addition to the CPS version. The choice of running the CPS version is delayed to run time. Since CPS code is usually slower, this can avoid degradations. In addition, one can ensure that some code is run in direct style by using {{Jsoo_runtime.Effect.assume_no_perform}}. A caveat is that Dune does not know about {{--effects=double-translation}} yet and may try to link together files built with {{--enable=double-translation}} and files built with only {{--enable=effects}}, which gives an error. As long as this is not fixed, running {{dune clean}} should help.
 
 === Dune integration ===
 

manual/overview.wiki

@@ -85,7 +85,7 @@ functions are optimized:
 <<a_manual chapter="tailcall" |More about tail call optimization>>.
 
 Effect handlers are fully supported with the
-{{{--effects={cps,double-translation}}}} flag. Effect support is disabled by
+{{{--enable=effects}}} flag. Effect support is disabled by
 default for now since effects are not widely used at the moment and the
 generated code can be slower, larger and less readable. See the dedicated
 manual section about effects for details.