Improve _element_constructor_ of image set

[user202729]

Allow the example shown to work.

        sage: R.<x> = QQ[]
        sage: S.<y> = QQ[]
        sage: R.hom([y^2]).inverse_image(y^4)
        x^2
        sage: y^4 in R.hom([y^2]).image()
        True

Also mention that it is not recommended to pass inverse=... to ImageSubobject constructor — there should be no need for this, it is better (and more discoverable) to define the .inverse_image method on the map.

I could have deprecated it, but decide to leave it there just in case. The only place within the doctest that it is used is in the file itself.

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