[3.14] gh-133412: amend docs for the `inst` definition (GH-133708) (#133729)

gh-133412: amend docs for the `inst` definition (GH-133708)

The `stack_effect` is incorrectly documented as being allowed to be optional.
(cherry picked from commit f77dac66e1867cfdf6f7549822dd64416f6063bc)

Co-authored-by: Nybblista <170842536+nybblista@users.noreply.github.com>