gotest: document -test.parallel

R=golang-dev, bradfitz, dsymonds, gri
CC=golang-dev
https://golang.org/cl/5223043

src/cmd/gotest/doc.go

@@ -54,6 +54,7 @@ Usage:
 	6.out [-test.v] [-test.run pattern] [-test.bench pattern] \
 		[-test.cpuprofile=cpu.out] \
 		[-test.memprofile=mem.out] [-test.memprofilerate=1] \
+		[-test.parallel=0] \
 		[-test.timeout=10] [-test.short] \
 		[-test.benchtime=3] [-test.cpu=1,2,3,4]
 
@@ -86,6 +87,10 @@ collection.
 Use -test.run or -test.bench to limit profiling to a particular test
 or benchmark.
 
+The -test.parallel flag allows parallel execution of Test functions
+that call test.Parallel.  The value of the flag is the maximum number
+of tests to run simultaneously; by default, parallelism is disabled.
+
 The -test.short flag tells long-running tests to shorten their run
 time.  It is off by default but set by all.bash so installations of
 the Go tree can do a sanity check but not spend time running