go: add comment on why external linking is needed (bazel-contrib#2630)

For bazel-contrib#2614

Author: Jay Conrod

go/private/actions/link.bzl

@@ -77,6 +77,15 @@ def emit_link(
     if ((go.mode.static and not go.mode.pure) or
         go.mode.link != LINKMODE_NORMAL or
         go.mode.goos == "windows" and (go.mode.race or go.mode.msan)):
+        # Force external linking for the following conditions:
+        # * Mode is static but not pure: -static must be passed to the C
+        #   linker if the binary contains cgo code. See #2168, #2216.
+        # * Non-normal build mode: may not be strictly necessary, especially
+        #   for modes like "pie".
+        # * Race or msan build for Windows: Go linker has pairwise
+        #   incompatibilities with mingw, and we get link errors in race mode.
+        #   Using the C linker avoids that. Race and msan always require a
+        #   a C toolchain. See #2614.
         tool_args.add("-linkmode", "external")
     if go.mode.static:
         extldflags.append("-static")