Does the || operator evaluate the second argument even if the first argument is true?

I'm trying to evaluate the expression (a=10) || (rr=20) while the rr variable is not defined

so typing rr in the ruby console before evaluating the previous expression returns

rr
NameError: undefined local variable or method `rr' for main:Object
from (irb):1
from :0

When I write the expression (a=10) || (rr=20) it returns 10, and when I write rr afterwards it says nil

(a=10) || (rr=20)
rr  # => nil

so, why is this happening? Shouldn't rr be defined only if the second argument of the || operator is evaluated, which should be never based on the documentation?