bus/call: Clarify error sources

There is some confusion, likely created by me, that call() might not
throw an exception in the case of a method call returning a
METHOD_ERROR.

This commit clarifies that all errors are thrown, and that we do not
need redundant is_method_error() checks on the reesulting message.

Change-Id: Ic2b962961cf671bd562bce7ba47135ad89bf9371
Signed-off-by: William A. Kennington III <wak@google.com>

sdbusplus/bus.hpp.in

@@ -234,6 +234,10 @@ struct bus
     }
 
     /** @brief Perform a message call.
+     *         Errors generated by this call come from underlying dbus
+     *         related errors *AND* from any method call that results
+     *         in a METHOD_ERROR. This means you do not need to check
+     *         is_method_error() on the returned message.
      *
      *  @param[in] m - The method_call message.
      *  @param[in] timeout_us - The timeout for the method call.