Operator precedence is documented. Operators without built-in meanings are documented.
The word "associative" apparently does not appear in either page. It's easy enough to do an experiment to observe the associativity of an operator. For example,
\[CircleMinus] is left-associative and
\[CirclePlus] is not:
ClearAll[CirclePlus, CircleMinus]; CircleMinus[a_, b_] := a[b]; CirclePlus[a_, b_] := a[b]; a⊖b⊖c a⊕b⊕c (a⊕b)⊕c
a[b][c] a⊕b⊕c a[b][c]
But it would be nicer to discover associativity without an experiment. Neither operator has
Attributes, let alone attributes that reveal associativity.
In:= ClearAll[CirclePlus, CircleMinus]; Attributes[CircleMinus] Attributes[CirclePlus]