Perform the following changes:
- change definition of `void Mutex::lock(void)` to `osStatus Mutex::lock(void)`.
- change definition of `void Mutex::unlock()` to `osStatus Mutex::unlock()`.
- use MBED_ERROR1 macro to check the lock/unlock operation status.
- add notes in the description of lock/unlock functions: "This function asserts status of the lock/unlock operation (will not return in case of failure). Use of the return value is deprecated, as the return is expected to become void in the future.".
- modify/add description of the return value.
- remove reference to Mbed 6.
- make `lock(millisec)` deprecated in favour of lock(), trylock() and trylock_for() functions.