doc: expand os.loadavg() section

Add a short explanation of what the load average is and why it's
unavailable on Windows.

Also sneak in a fix for a typo that I introduced in commit 56c5806.

doc/api/os.markdown

@@ -41,7 +41,14 @@ Returns the system uptime in seconds.
 ## os.loadavg()
 
 Returns an array containing the 1, 5, and 15 minute load averages.
-This function always return `[0, 0, 0]` on Windows.
+
+The load average is a measure of system activity, calculated by the operating
+system and expressed as a fractional number.  As a rule of thumb, the load
+average should ideally be less than the number of logical CPUs in the system.
+
+The load average is a very UNIX-y concept; there is no real equivalent on
+Windows platforms.  That is why this function always returns `[0, 0, 0]` on
+Windows.
 
 ## os.totalmem()