I'd really like to read an explicit discussion of this in the official documentation, rather than just glean what I can from answers to questions.
The official documentation cannot have a dissertation on every combination of hardware, OS, file-system type, version of that file-system, and your usage pattern. That is inherently the realm of the wiki or the blogs.
The documentation has enough information at this level of detail that I wouldn't object to adding commentary addressing the above should someone take the time to write it.
Given that the location of pg_xlog is not "configurable" placing such commentary in Server Configuration would be a no-go, however. At a quick glance a new section under "Server Setup and Operation - Creating a Database Cluster" would probably be a better home. It already discusses Secondary File Systems and in many ways this is just an extension of that discussion.