chgserver: update comment describing when to fall back to core _runsystem() stable

chgserver: update comment describing when to fall back to core _runsystem()

The condition "output needs to be captured" was moved to the core ui, and
removed at fbce78c58f1e "chg: refactor ui.system() to be partly overridden."

The next patch will add one more "if" to address the issue 5992.

--- a/mercurial/chgserver.py	Wed Sep 26 20:53:59 2018 +0900
+++ b/mercurial/chgserver.py	Wed Sep 26 21:21:05 2018 +0900
@@ -198,10 +198,10 @@
                 self._csystem = csystem
 
         def _runsystem(self, cmd, environ, cwd, out):
-            # fallback to the original system method if the output needs to be
-            # captured (to self._buffers), or the output stream is not stdout
-            # (e.g. stderr, cStringIO), because the chg client is not aware of
-            # these situations and will behave differently (write to stdout).
+            # fallback to the original system method if
+            #  a. the output stream is not stdout (e.g. stderr, cStringIO),
+            # because the chg client is not aware of these situations and
+            # will behave differently (i.e. write to stdout).
             if (out is not self.fout
                 or not util.safehasattr(self.fout, 'fileno')
                 or self.fout.fileno() != procutil.stdout.fileno()):