[mlir][tensor] Document `dest` operand (#71726)

Based on the tips from @ubfx and @joker-eph in
https://github.com/llvm/llvm-project/issues/70030

, this patch suggest to
introduce the `dest` operand in the `tensor` dialect description. To do
so, this patch also suggests to move some things around to make it more
clear how the paragraphs relate to each other.

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Co-authored-by: Matthias Springer <me@m-sp.org>
Co-authored-by: Mehdi Amini <joker.eph@gmail.com>
Co-authored-by: Felix Schneider <fx.schn@gmail.com>