explain tracepoint extensions, drop InstallInTrace

src/stub/core_impl/base.rs

@@ -173,7 +173,22 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
                 }
 
                 if let Some(_ops) = target.support_tracepoints() {
-                    res.write_str(";InstallInTrace+")?;
+                    // There are a number of optional tracepoint extensions that
+                    // gdbstub should eventually implement.
+                    // * `StaticTracepoint` for static tracepoint support.
+                    // * `EnableDisableTracepoints` for enabling/disabling tracepoints during a
+                    //   trace experiment.
+                    // * `tracenz` for the tracenz agent bytecode operation.
+                    // * The `Qbtrace:*` family for branch tracing.
+                    // * `InstallInTrace` allows for gdbstub to deliver tracepoint configuration
+                    //   commands while the trace experiment is running instead of them only taking
+                    //   affect on the next `tstart` command.
+                    //
+                    // For now, gdbstub doesn't provide trait extensions for these
+                    // options and so we don't report support. We do report support
+                    // for one extension however:
+                    // * `QTBuffer:size` for configuring the trace buffer size, since
+                    //   the target is allowed to implement it as a no-op.
                     res.write_str(";QTBuffer:size+")?;
                 }