Add doc notes about collpasing multiline string

Author: harendra-kumar

core/src/Streamly/Unicode/String.hs

@@ -7,6 +7,10 @@
 -- Portability : GHC
 --
 -- Convenient template Haskell quasiquoters to format strings.
+--
+-- The 'str' quasiquoter retains newlines in the string when the line is split
+-- across multiple lines. The @unwords . lines@ idiom can be used on the
+-- resulting string to collapse it into a single line.
 
 module Streamly.Unicode.String
     ( str