[MLIR][Math] Improve docs (NFC)

Remove boilerplate examples and add a text at the dialect level to describe
what kind of operands the operations accept (i.e., scalar, tensor or vector).
Left a shorter sentence describing the input operands for each operation as
this redundancy is convenient when browsing the documentation using the
website.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D126648