Clarify probe documentation.

Fixes #2510

Submitted by:	klean@github

doc/sphinx/reference/vcl.rst

@@ -270,11 +270,12 @@ There are no mandatory options. These are the options you can set:
 
   ``.url``
     The URL to query. Defaults to ``/``.
+    Mutually exclusive with ``.request``
 
   ``.request``
     Specify a full HTTP request using multiple strings. ``.request`` will
-    have ``\r\n`` automatically inserted after every string. If
-    specified, ``.request`` will take precedence over ``.url``.
+    have ``\r\n`` automatically inserted after every string. 
+    Mutually exclusive with ``.url``.
 
   ``.expected_response``
     The expected HTTP response code. Defaults to ``200``.
@@ -296,8 +297,9 @@ There are no mandatory options. These are the options you can set:
     Defaults to ``8``.
 
   ``.threshold``
-    How many of the polls in .window must have succeeded for us to
-    consider the backend healthy. Defaults to ``3``.
+    How many of the polls in ``.window`` must have succeeded to
+    for the backend to be healthy.
+    Defaults to ``3``.
 
 
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