I2C: document concurrent access

[ysoldak]

Use of I2C in TinyGo is not "thread-safe".
HAL would not use mutex to serialise concurrent access. Synchronisation when needed, must be done in application code.

This is contra, for example, to Linux I2C driver that uses internal mutex to enable easy concurrent usage.

Arguments against implementing such internal mutex in TinyGo (from slack):

• it is usually not necessary [in embedded systems] and thus introduces overhead
• easy to add but impossible to remove (for backwards compatibility reasons)

Page: https://tinygo.org/docs/concepts/peripherals/i2c/

[aykevl]

Note that in the case of Linux, the kernel has to synchronize access to the I2C bus for safe operation. I'm pretty sure the hardware will misbehave when access isn't synchronized which would be unacceptable when done from an unprivileged process. So I think it's also a security feature, not just a convenience feature. This security argument doesn't apply to TinyGo.

(This is a bit of speculation though...)