sshpeer: move docstring to top

sshpeer: move docstring to top

Also makes it use double quotes consistently.

--- a/mercurial/sshpeer.py	Tue Dec 19 21:41:39 2017 +0900
+++ b/mercurial/sshpeer.py	Mon Dec 18 21:15:53 2017 +0900
@@ -18,9 +18,9 @@
 )
 
 def _serverquote(s):
+    """quote a string for the remote shell ... which we assume is sh"""
     if not s:
         return s
-    '''quote a string for the remote shell ... which we assume is sh'''
     if re.match('[a-zA-Z0-9@%_+=:,./-]*$', s):
         return s
     return "'%s'" % s.replace("'", "'\\''")