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>