Configure deadline with markdown.parseTimeout

Allow site admin to increase the deadline from the default of 2s
to higher values, allowing more time to parse complex documents.

Change-Id: I0189885c84424218f6ee83d7475cb4c9eac44889

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index bed3fe7..e1c6da3 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -768,6 +768,17 @@
   imageLimit = 256K
 ```
 
+### Parsing timeout
+
+Parsing Markdown can be expensive so this implementation places
+a default upper bound of 2 seconds on running time per document.
+This is measured in wall clock time from the start of the request.
+
+```
+[markdown]
+  parseTimeout = 2s
+```
+
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