loggingutil: document openlogfile()

loggingutil: document openlogfile()

This function will be used later for command-server logging.

--- a/mercurial/loggingutil.py	Sun Nov 18 18:25:37 2018 +0900
+++ b/mercurial/loggingutil.py	Sun Nov 18 18:35:31 2018 +0900
@@ -15,6 +15,10 @@
 )
 
 def openlogfile(ui, vfs, name, maxfiles=0, maxsize=0):
+    """Open log file in append mode, with optional rotation
+
+    If maxsize > 0, the log file will be rotated up to maxfiles.
+    """
     def rotate(oldpath, newpath):
         try:
             vfs.unlink(newpath)