Fix documentation for parse_performance_data DSL function

The function does not support parsing multiple values at once but instead
generates labels like "foo=1 bar":

  Icinga 2 (version: v2.13.0-729-g8fe957bc2)
  Type $help to view available commands.
  <1> => parse_performance_data("foo=1 bar=2")
  {
  	counter = false
  	crit = null
  	label = "foo=1 bar"
  	max = null
  	min = null
  	type = "PerfdataValue"
  	unit = ""
  	value = 2.000000
  	warn = null
  }

While a function with the documented behavior would probably be more useful,
that change can't be made in a compatible way as this would have to change the
return type from PerfdataValue to Array.

doc/18-library-reference.md

@@ -446,7 +446,8 @@ Signature:
 function parse_performance_data(pd)
 ```
 
-Parses a performance data string and returns an array describing the values.
+Parses a single performance data value from a string and returns a
+[PerfdataValue](08-advanced-topics.md#advanced-value-types-perfdatavalue) object.
 
 Example: