CROSSTOOLS: Fix comment for /EHsc compiler flag

The previous comment referred to the option `/EHa`, but `/EHsc` is the option to only catch C++ standards (which seems to be what we want, according to this doc: https://msdn.microsoft.com/en-us/library/1deeycx5.aspx)

Closes #4739.

PiperOrigin-RevId: 187593737

tools/cpp/CROSSTOOL.tpl

diff --git a/tools/cpp/CROSSTOOL.tpl b/tools/cpp/CROSSTOOL.tpl
index 57895e5..dce98cd 100644
--- a/tools/cpp/CROSSTOOL.tpl
+++ b/tools/cpp/CROSSTOOL.tpl
@@ -249,7 +249,8 @@
   compiler_flag: "/Gy"
   # Use string pooling.
   compiler_flag: "/GF"
-  # Catch both asynchronous (structured) and synchronous (C++) exceptions.
+  # Catch C++ exceptions only and tell the compiler to assume that functions declared
+  # as extern "C" never throw a C++ exception.
   compiler_flag: "/EHsc"
 
   # Globally disabled warnings.