Update doc for ssh URL, describing path handling more clearly.

doc/hg.1.txt

@@ -413,8 +413,10 @@
       https://[user@]host[:port][/path]
       ssh://[user@]host[:port][/path]
 
-    SSH requires an accessible shell account on the destination
-    machine and a copy of hg in the remote path.
+    SSH requires an accessible shell account on the destination machine
+    and a copy of hg in the remote path.  With SSH, paths are relative
+    to the remote user's home directory by default; use two slashes at
+    the start of a path to specify it as relative to the filesystem root.
 
     options:
     -u, --update   update the working directory to tip after pull