sync: clarify RwLock fairness documentation

[cluster2600]

Fixes #6901

Problem

The previous wording in the RwLock documentation was misleading:

if a task that wishes to acquire the write lock is at the head of the queue, read locks will not be given out until the write lock has been released

This implies that readers are only blocked when the writer is at position #1 in the queue. However, due to the FIFO ordering policy, any write request queued before a read request will block that reader — the writer's position (head or not) is irrelevant from the reader's perspective.

Example: If R1 → W → R2 are queued in that order, R2 is blocked by W even though W is not at the head of the queue. Once R1 gets the lock (it was already queued before W), R2 cannot skip past W.

Fix

Replace with a more accurate description:

a read lock will not be given out until all write lock requests that were queued before it have been acquired and released

This clearly communicates the FIFO semantics without the misleading "head of the queue" implication.

This rephrasing was suggested in the issue by @cip999 and acknowledged as reasonable by @Darksonn.

[Darksonn]

This seems ok to me. Thanks!