doc: add message clarifying linter rule disable

.clang-tidy

@@ -25,6 +25,7 @@ Checks: >
 #  TODO(someone): clean up misc-non-private-member-variables-in-classes and add option back in
 # Not using google-readability-avoid-underscore-in-googletest-name because it also fails for test_name
 # Not using abseil-string-find-str-contains because we don't want to include more libraries
+# Not using bugprone-unused-return-value because we get spurious positives for overridden 'T& operator=(T&)'
 CheckOptions:
   - { key: readability-identifier-naming.NamespaceCase,          value: lower_case }
   - { key: readability-identifier-naming.ClassCase,              value: CamelCase }