selftests/bpf: Improve task local data documentation and fix potential memory leak

selftests/bpf: Improve task local data documentation and fix potential memory leak

If TLD_FREE_DATA_ON_THREAD_EXIT is not enabled in a translation unit
that calls __tld_create_key() first, another translation unit that
enables it will not get the auto cleanup feature as pthread key is only
created once when allocation metadata. Fix it by always try to create
the pthread key when __tld_create_key() is called.

Also improve the documentation:
- Discourage user from using different options in different translation
  units
- Specify calling tld_free() before thread exit as undefined behavior

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260331213555.1993883-6-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>