api-trace2.txt: print config key-value pair

It's supported to print "interesting" config key-value paire
to tr2 log by setting "GIT_TRACE2_CONFIG_PARAMS" environment
variable and the "trace2.configparam" config, let's add the
related docs in Documentaion/technical/api-trace2.txt.

Signed-off-by: Teng Long <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dyrone

Documentation/technical/api-trace2.txt

@@ -1207,6 +1207,37 @@ at offset 508.
 This example also shows that thread names are assigned in a racy manner
 as each thread starts and allocates TLS storage.
 
+Config (def param) Events::
+
+	  Dump "interesting" config values to trace2 log.
++
+We can optionally emit configuration events, see
+`trace2.configparams` in linkgit:git-config[1] for how to enable
+it.
++
+----------------
+$ git config color.ui auto
+----------------
++
+Then, mark the config `color.ui` as "interesting" config with
+`GIT_TRACE2_CONFIG_PARAMS`:
++
+----------------
+$ export GIT_TRACE2_PERF_BRIEF=1
+$ export GIT_TRACE2_PERF=~/log.perf
+$ export GIT_TRACE2_CONFIG_PARAMS=color.ui
+$ git version
+...
+$ cat ~/log.perf
+d0 | main                     | version      |     |           |           |              | ...
+d0 | main                     | start        |     |  0.001642 |           |              | /usr/local/bin/git version
+d0 | main                     | cmd_name     |     |           |           |              | version (version)
+d0 | main                     | def_param    |     |           |           |              | color.ui:auto
+d0 | main                     | data         | r0  |  0.002100 |  0.002100 | fsync        | fsync/writeout-only:0
+d0 | main                     | data         | r0  |  0.002126 |  0.002126 | fsync        | fsync/hardware-flush:0
+d0 | main                     | exit         |     |  0.002142 |           |              | code:0
+d0 | main                     | atexit       |     |  0.002161 |           |              | code:0
+----------------
 == Future Work
 
 === Relationship to the Existing Trace Api (api-trace.txt)