Document that Get*MappedRange is not thread-safe with Unmap

[kainino0x]

From a discussion with @lokokung about making mapping thread-safe in Dawn (without taking device-wide locks).

GetMappedRange()/GetConstMappedRange() return raw pointers which means that calling code always needs to be thread-safe w.r.t. Unmap(). For example this can never be safe:

• Thread 1: p = GetMappedRange(); mutex A { use p }
• Thread 2: mutex A { Unmap() }

because the unmap could happen between GetMappedRange and a lock being taken on A. The only way to be safe is to include GetMappedRange in the lock:

• Thread 1: mutex A { p = GetMappedRange(); use p }
• Thread 2: mutex A { Unmap() }

Thus, I think there is no use in having GetMappedRange be thread-safe w.r.t. Unmap.

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GetMappedRange should still be safe w.r.t. MapAsync, as you could do something like:

• Thread 1: MapAsync()
• Thread 2: p = GetMappedRange(); if (p) { do something if the map happens to be complete; Unmap() }

(even though there are alternatives like using GetMapState or polling the MapAsync future).

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Aside: technically it's not very useful for MapAsync and Unmap to be thread-safe w.r.t. one another, but I think they still should be, as they're not as hot.

[kainino0x]

Oops, realized my examples were a bit wrong and the justification is even stronger than I originally wrote. Fixed.