tr2tls: clarify TLS terminology

Reduce or eliminate use of the term "TLS" in the Trace2 code.

The term "TLS" has two popular meanings: "thread-local storage" and
"transport layer security".  In the Trace2 source, the term is associated
with the former.  There was concern on the mailing list about it refering
to the latter.

Update the source and documentation to eliminate the use of the "TLS" term
or replace it with the phrase "thread-local storage" to reduce ambiguity.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2.c

@@ -52,7 +52,7 @@ static struct tr2_tgt *tr2_tgt_builtins[] =
  * Force (rather than lazily) initialize any of the requested
  * builtin TRACE2 targets at startup (and before we've seen an
  * actual TRACE2 event call) so we can see if we need to setup
- * the TR2 and TLS machinery.
+ * private data structures and thread-local storage.
  *
  * Return the number of builtin targets enabled.
  */