time: document an alternative way of setting the system's time zone

User's program was mutating time.Local variable and crashing
itself as a consequence. Instead of documenting that time.Local
variable should not be mutated, recommended way of setting the
system's time zone has been documented.

Fixes #34814

Change-Id: I7781189855c3bf2ea979dfa07f86c283eed27091
Reviewed-on: https://go-review.googlesource.com/c/go/+/200457Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/zoneinfo.go

@@ -65,6 +65,11 @@ var UTC *Location = &utcLoc
 var utcLoc = Location{name: "UTC"}
 
 // Local represents the system's local time zone.
+// On Unix systems, Local consults the TZ environment
+// variable to find the time zone to use. No TZ means
+// use the system default /etc/localtime.
+// TZ="" means use UTC.
+// TZ="foo" means use file foo in the system timezone directory.
 var Local *Location = &localLoc
 
 // localLoc is separate so that initLocal can initialize