docs: `expect_s4_class()` cannot accept a vector of classes

[maelle]

The manual page for expect_s4_class() and expect_s3_class() states that the class argument can be

a character vector of class names

However it is not true for expect_s4_class() that uses

https://github.com/r-lib/testthat/blob/fe38519d72247a8907228a4ecaf926483aa5d4ff/R/expect-inheritance.R#L112C9-L112C20

where methods::is() only accepts one class.

I suspect using more than one class in expect_s4_class() might not make any sense 😅 but maybe the documentation of the parameter could be rephrased slightly?