Add brief warning about timeout best-effort

Author: polyfractal

060_Distributed_Search/15_Search_options.asciidoc

@@ -18,7 +18,7 @@ the _bouncing results_ problem.((("bouncing results problem")))
 .Bouncing Results
 ****
 
-Imagine that you are sorting your results by a `timestamp` field, and 
+Imagine that you are sorting your results by a `timestamp` field, and
 two documents have the same timestamp.  Because search requests are
 round-robined between all available shard copies, these two documents may be
 returned in one order when the request is served by the primary, and in
@@ -58,6 +58,22 @@ partial response with the `timed_out` property:
 --------------------------------------------------
 <1> The search request timed out.
 
+[WARNING]
+====
+It's important to know that the timeout is still a best-effort operation; it's
+possible for the query to surpass the allotted timeout.  There are two reasons for
+this behavior:
+
+1. Timeout checks are performed on a per-document basis.  However, some query types
+have a significant amount of work that must be performed *before* documents are evaluated.
+This "setup" phase does not consult the timeout, and so very long setup times can cause
+the overall latency to shoot past the timeout.
+2. Because the time is once per document, a very long query can execute on a single
+document and it won't timeout until the next document is evaluated.  This also means
+poorly written scripts (e.g. ones with infinite loops) will be allowed to execute
+forever.
+====
+
 [[search-routing]]
 ==== routing
 
@@ -79,7 +95,7 @@ discuss it in detail in <<scale>>.
 ==== search_type
 
 The default search type is `query_then_fetch` ((("query_then_fetch search type")))((("search options", "search_type")))((("search_type"))). In some cases, you might want to explicitly set the `search_type`
-to `dfs_query_then_fetch` to improve the accuracy of relevance scoring: 
+to `dfs_query_then_fetch` to improve the accuracy of relevance scoring:
 
 [source,js]
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