Add a comment at the top of export_template.h explaining the goal of all the template magic.

These macros are used in all of 3 files. Instead of all this magic, maybe the components
that need this should define FOO_TEMPLATE_EXPORT_DECLARE and FOO_TEMPLATE_EXPORT_DEFINE
in their foo_export.h file instead, and make it expand to the right thing there.
That seems a lot simpler and easier to understand.

But for now, let's just add a comment.

Bug: none
Change-Id: I5fd56440a07a4c7ba879af6d809c7e1397d05737
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890531
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710908}

base/export_template.h

@@ -26,6 +26,18 @@
 
 // Implementation notes
 //
+// On Windows, when building the FOO library (that is, when FOO_EXPORT expands
+// to __declspec(dllexport)), we want the two lines to expand to:
+//
+//     extern template class foo<bar>;
+//     template class FOO_EXPORT foo<bar>;
+//
+// In all other cases (non-Windows, and Windows when using the FOO library (that
+// is when FOO_EXPORT expands to __declspec(dllimport)), we want:
+//
+//     extern template class FOO_EXPORT foo<bar>;
+//     template class foo<bar>;
+//
 // The implementation of this header uses some subtle macro semantics to
 // detect what the provided FOO_EXPORT value was defined as and then
 // to dispatch to appropriate macro definitions.  Unfortunately,