doc: fix typo in example for function string_eval_expression (plugin API reference)
core: fix priority of logical operators in evaluation of expression

The AND ("&&") takes precedence over the OR ("||").

Before the fix:
>> 1 || 1 && 0
== [0]

After the fix:
>> 1 || 1 && 0
== [1]

Since the "&&" has higher priority, expression is evaluated as:
"1 || (1 && 0)".