doc(data/finset/basic): rewrite finset module doc (#5893)
doc(data/finset/basic): rewrite finset module doc (#5893)



Co-authored-by: Bryan Gin-ge Chen <6e05157ce10643a7cfc5a3d4e29e4fa5cffe1fc6@gmail.com>
Co-authored-by: Hanting Zhang <7a284909f2b7b89d1927eb7a2d1972dbc972ca9f@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib