cmd/go: Document that -cover causes incorrect line numbers

Due to the fact that -cover injects additional code to the original
source, tests run with -cover will often have incorrect line numbers.

Also includes docs for -list regexp missed by ba8ff87d

Updates #6329

Change-Id: I87f0618ac31e96071bca61055cc17c0cbdee208a
Reviewed-on: https://go-review.googlesource.com/38640Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/alldocs.go

@@ -1437,6 +1437,9 @@
 // 	-cover
 // 	    Enable coverage analysis.
 //
+// 	    BUG: If a compilation or test fails with coverage enabled,
+// 	    the reported line numbers may be incorrect.
+//
 // 	-covermode set,count,atomic
 // 	    Set the mode for coverage analysis for the package[s]
 // 	    being tested. The default is "set" unless -race is enabled,
@@ -1459,6 +1462,11 @@
 // 	    benchmarks should be executed.  The default is the current value
 // 	    of GOMAXPROCS.
 //
+// 	-list regexp
+// 	    List tests, benchmarks, or examples matching the regular expression.
+// 	    No tests, benchmarks or examples will be run. This will only
+// 	    list top-level tests. No subtest or subbenchmarks will be shown.
+//
 // 	-parallel n
 // 	    Allow parallel execution of test functions that call t.Parallel.
 // 	    The value of this flag is the maximum number of tests to run

src/cmd/go/internal/test/test.go

@@ -162,6 +162,9 @@ const testFlag2 = `
 	-cover
 	    Enable coverage analysis.
 
+	    BUG: If a compilation or test fails with coverage enabled,
+	    the reported line numbers may be incorrect.
+
 	-covermode set,count,atomic
 	    Set the mode for coverage analysis for the package[s]
 	    being tested. The default is "set" unless -race is enabled,