Clarify search load limit description for users unfamiliar with Unix load average

Support topic: https://www.phpbb.com/community/viewtopic.php?f=46&t=2137271

The setting is at General -> Server configuration -> Search settings -> Search page system load limit.

The following help text is currently provided:

If the 1 minute system load exceeds this value the search page will go offline, 1.0 equals ~100% utilisation of one processor. This only functions on UNIX based servers.

Users who are unfamiliar with how Unix load average works may assume (from the provided help text, especially if they have a single-core machine) that 1 is the highest load average possible, and therefore would be tempted to set it to something less than 1. In reality a load average of 1 is not high at all, and most users would want to set the limit quite a bit higher than than.

We could link to wikipedia somewhere in the help text (http://en.wikipedia.org/wiki/Load_%28computing%29#Unix-style_load_calculation) and/or provide a more comprehensive explanation of how the load average works and/or provide a disclaimer for the user to research what the load average really is before changing the value from default.