Deprecate '_' in YAML

Mixing '-' and '_' for keywords in the same file can be confusing.
Deprecate remaining instances of '_' in favor of alternatives that
use '-'.

Change-Id: I1e8af0d5396dd24b45b024a2897f61f8745a8e1f
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 06a85bc..cb49c82 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -792,19 +792,28 @@
   *expiry* (optional)
     How long the signed destination should be available for
 
-  *max_file_size** (optional)
+  *max-file-size** (optional)
     The maximum size of an individual file
 
-  *max_file_count* (optional)
+  *max_file_size** (optional, deprecated)
+    A deprecated alternate spelling of *max-file-size*.
+
+  *max-file-count* (optional)
     The maximum number of separate files to allow
 
-  *logserver_prefix*
+  *max_file_count* (optional, deprecated)
+    A deprecated alternate spelling of *max-file-count*.
+
+  *logserver-prefix*
     Provide a URL to the CDN or logserver app so that a worker knows
     what URL to return.
     ``For example: http://logs.example.org/server.app?obj=``
-    The worker should return the logserver_prefix url and the object
+    The worker should return the logserver-prefix url and the object
     path as the URL in the results data packet.
 
+  *logserver_prefix* (deprecated)
+    A deprecated alternate spelling of *logserver-prefix*.
+
 Here is an example of setting the failure message for jobs that check
 whether a change merges cleanly::