doc: document build timeouts in FAQ

Fix fedora-copr#1976

doc/user_documentation.rst

@@ -859,6 +859,22 @@ contents of the directories.  Either please ignore the inconsistency, or visit
 the `non-cached host variant
 <http://copr-be.cloud.fedoraproject.org/results/>`_.
 
+.. _`faq-build-timeout`:
+
+.. rubric:: My build failed because of a timeout, why?  :ref:`¶ <faq-build-timeout>`
+
+Builds are not allowed to run forever. The default limit is 5 hours
+(18000 seconds) but users can increase it up to 30 hours (108000
+seconds). If the timeout limit is exceeded, the build will be killed
+with the following error message:
+
+::
+
+    !! Copr timeout => sending INT
+    Copr build error: Build failed
+    Shared connection to builder closed.
+
+
 .. _`Weird SCM build failure?`:
 
 .. rubric:: Weird SCM build failure? :ref:`¶ <Weird SCM build failure?>`