minor #10433 Add note on Apache issue with FallbackResource (pavlakis)

This PR was submitted for the 4.1 branch but it was merged into the 4.3 branch instead (closes #10433).

Discussion
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Add note on Apache issue with FallbackResource

There was an Apache bug with FallbackResource when being called on the home page `/` causing Apache to hang. This is a difficult issue to debug and hence would be helpful to add a note here.
The Apache bug details can be found here: https://bz.apache.org/bugzilla/show_bug.cgi?id=58292

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Commits
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0f7a705 Add note on Apache issue with FallbackResource

Author: javiereguiluz

setup/web_server_configuration.rst

@@ -124,6 +124,10 @@ and increase web server performance:
         #SetEnv DATABASE_URL "mysql://db_user:db_pass@host:3306/db_name"
     </VirtualHost>
 
+.. caution::
+
+Use ``FallbackResource`` on Apache 2.4.25 or higher, due to a bug which was fixed on that release causing the root ``/`` to hang.
+
 .. tip::
 
     If you are using **php-cgi**, Apache does not pass HTTP basic username and