Mention .coerce() method in coercion tour

[user202729]

I feel it's too hard to discover previously.

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[fchapoton]

Would you please break the very long lines you add in the doctests ?

[user202729]

speaking of which there are two inconsistent styles

1. Add one space of indent

    Notice that the zero tensor field is immutable, and therefore its
    components cannot be changed::

        sage: zer = M.tensor_field_module((1, 1)).zero()
        sage: zer.is_immutable()
        True
        sage: zer.set_comp()
        Traceback (most recent call last):
        ...
        ValueError: the components of an immutable element cannot be
         changed

e.g. src/sage/manifolds/differentiable/tensorfield.py

2. Doesn't

        NotImplementedError: is_square() not implemented for
        non-constant or relational elements of Symbolic Ring

e.g. src/sage/arith/misc.py

Just nitpicking.