Clarify wording of the Sync requirement for an immutable static

Additionally, this drops the statement that all access is "safe" since
there isn't anything that would imply that it is unsafe.

Author: ehuss

src/items/static-items.md

@@ -22,8 +22,7 @@ Static initializers may refer to other statics.
 Non-`mut` static items that contain a type that is not [interior mutable] may
 be placed in read-only memory.
 
-All access to a static is safe,
-provided that the type must have the `Sync` trait bound to allow thread-safe access.
+The type of an immutable static must implement the [`Sync`](std::marker::Sync) trait.
 
 The initializer expression must be omitted in an [external block], and must be
 provided for free static items.