Add a comment explaining the buffer binding points.

src/libANGLE/renderer/gl/BufferGL.cpp

@@ -16,7 +16,14 @@
 namespace rx
 {
 
+// Use the GL_COPY_READ_BUFFER binding when two buffers need to be bound simultaneously.
+// GL_ELEMENT_ARRAY_BUFFER is supported on more versions but can modify the state of the currently
+// bound VAO.  Two simultaneous buffer bindings are only needed for glCopyBufferSubData which also
+// adds the GL_COPY_READ_BUFFER binding.
 static const GLenum SourceBufferOperationTarget = GL_COPY_READ_BUFFER;
+
+// Use the GL_ELEMENT_ARRAY_BUFFER binding for most operations since it's available on all
+// supported GL versions and doesn't affect any current state when it changes.
 static const GLenum DestBufferOperationTarget = GL_ARRAY_BUFFER;
 
 BufferGL::BufferGL(const FunctionsGL *functions, StateManagerGL *stateManager)