Add explanatory note to docstring for pw_hash

mention the meaning of the first two characters as a strength parameter

Author: sabo

lib/puppet/parser/functions/pw_hash.rb

@@ -25,7 +25,9 @@
   |bcrypt-x |2x    |bug compatible       |
   |bcrypt-y |2y    |historic alias for 2b|
 
-  The third argument to this function is the salt to use.
+  The third argument to this function is the salt to use. For bcrypt-type hashes,
+  the first two characters of the salt represent a strength parameter, with a value
+  between 4 and 31 inclusive.
 
   @return [String]
     Provides a crypt hash usable on most POSIX systems.