Update MemorySanitizer (MSan) documentation

I found a couple potential issues with the msan documentation:
https://www.chromium.org/developers/testing/memorysanitizer

1) I had an unexpected test crash, possibly related to outdated recommended gn args:
(my crash might have also been from skipping the GYP_DEFINES step at first)

Recommended:
 is_msan = true
 use_prebuilt_instrumented_libraries = true
 is_debug = false  # Release build.
 enable_nacl = false  # Necessary until the NaCl GN build is more complete.

Linux_MSan_Builder/Linux_ChromiumOS_MSan_Builder (non-trivial differences):
 +is_component_build = false
 +msan_track_origins = 2
 +strip_absolute_paths_from_debug_symbols = true
 -enable_nacl = false

2) The docs say to "Run the resulting binaries as usual. Pipe the output through tools/valgrind/asan/asan_symbolize.py to get symbolized reports. If running the "chrome" target pipe stderr and stdout."

That didn't work for me when running a browser_tests fixture. Running "out/Default/browser_tests --gtest_filter=DemoAppLauncherTest.Basic | tools/valgrind/asan/asan_symbolize.py" with my crash from https://crrev.com/502376 didn't  symbolize the stack. I had to copy the stack to a separate file (crash.txt) and pipe that via "cat crash.txt | tools/valgrind/asan/asan_symbolize.py"... Is that expected? (update docs if so, split out a bug if not?)