trace2: add variable description to git.txt

Documentation/technical/api-trace2.txt contains the full details
of the trace2 API and the GIT_TR2* environment variables. However,
most environment variables are included in Documentation/git.txt,
including the GIT_TRACE* variables.

Add a brief description of the GIT_TR2* variables with links to
the full technical details. The biggest difference from the
original variables is that we can specify a Unix Domain Socket.
Mention this difference, but leave the details to the technical
documents.

Reported-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -660,6 +660,27 @@ of clones and fetches.
 	When a curl trace is enabled (see `GIT_TRACE_CURL` above), do not dump
 	data (that is, only dump info lines and headers).
 
+`GIT_TR2`::
+	Enables more detailed trace messages from the "trace2" library.
+	Output from `GIT_TR2` is a simple text-based format for human
+	readability.
++
+The `GIT_TR2` variables can take many values. Any value available to
+the `GIT_TRACE` variables is also available to `GIT_TR2`. The `GIT_TR2`
+variables can also specify a Unix Domain Socket. See
+link:technical/api-trace2.html[Trace2 documentation] for full details.
+
+`GIT_TR2_EVENT`::
+	This setting writes a JSON-based format that is suited for machine
+	interpretation. See link:technical/api-trace2.html[Trace2 documentation]
+	for full details.
+
+`GIT_TR2_PERF`::
+	In addition to the text-based messages available in `GIT_TR2`, this
+	setting writes a column-based format for understanding nesting
+	regions. See link:technical/api-trace2.html[Trace2 documentation]
+	for full details.
+
 `GIT_REDACT_COOKIES`::
 	This can be set to a comma-separated list of strings. When a curl trace
 	is enabled (see `GIT_TRACE_CURL` above), whenever a "Cookies:" header