mcp: improve documentation of Add*Middleware

[jboolean]

This updates the documentation comments for AddReceivingMiddleware and AddSendingMiddleware to clarify how middlewares are ordered. The existing documentation is confusing and could lead one to believe middleware is applied from right to left at the time of execution (e.g. the mcp-go sdk does it this way).
In fact, because the wrapping is done at the time of adding and not at the time of execution, middleware is only applied in reverse order (and therefore run in order) within the set of middlewares being added together, while middleware added successively is applied on top of existing middleware and thus run in reverse order.