Add comment discussing Macro v.s. Turbo Assemblers

R=ahaas@chromium.org

Change-Id: Iffc27328464616b422cb6ccecbaf77d901f8d889
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2599848
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71912}

src/codegen/turbo-assembler.h

@@ -17,6 +17,12 @@ namespace internal {
 
 // Common base class for platform-specific TurboAssemblers containing
 // platform-independent bits.
+// You will encounter two subclasses, TurboAssembler (derives from
+// TurboAssemblerBase), and MacroAssembler (derives from TurboAssembler). The
+// main difference is that MacroAssembler is allowed to access the isolate, and
+// TurboAssembler accesses the isolate in a very limited way. TurboAssembler
+// contains all the functionality that is used by Turbofan, and does not expect
+// to be running on the main thread.
 class V8_EXPORT_PRIVATE TurboAssemblerBase : public Assembler {
  public:
   // Constructors are declared public to inherit them in derived classes