doc: specify thread safety for each data structure

The documentation of "Kernel/Data Structures" incorrectly
states that 'all provided structures are uniformly unsynchronized;
access to them is not threadsafe by default'.
In reality, some of them are threadsafe and some are not.
This might discourage users from using threadsafe data structures
where applicable, or mislead users into adding unnecessary locks.

This proposal addresses the issue by specifying thread safety
for each provided data structure.
My assessment of thread safety is based purely on source code analysis;
hence an expert verification would be appreciated.

Signed-off-by: Vukan Turkulov <vukant@gmail.com>

Author: Moon-Raven

doc/kernel/data_structures/index.rst

@@ -20,10 +20,12 @@ behind this design is to allow the collections to be used in contexts
 where dynamic allocation is disallowed (i.e. there is no need to
 allocate node objects because the memory is provided by the user).
 
-Note also that these libraries are uniformly unsynchronized; access to
+Note also that these libraries are generally unsynchronized; access to
 them is not threadsafe by default.  These are data structures, not
 synchronization primitives.  The expectation is that any locking
-needed will be provided by the user.
+needed will be provided by the user.  Some of the provided data
+structures are thread safe in specific usage scenarios (see
+:ref:`spsc_lockfree` and :ref:`mpsc_lockfree`).
 
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   :maxdepth: 1