style for inline comments in dynamically generated methods

Just a sync mail.

Dynamically generated methods have now inline comments that depict an
instance of the generated code:

    http://github.com/rails/rails/commit/a2270ef2594b97891994848138614657363f2806

That was inspired by wycats's:

    http://github.com/rails/rails/commit/6dc12881110d26bb952bd0f565623144f10a07b6

Please take that style into account :-). We iterated a bit over the
details on IRC. Comments are 2 spaces apart. If the line gets really
too long (say, beyond column 200) first we try to refactor the code to
shorten stuff. If not possible we move the comment above the *_eval
call.