Add docs for state and codepoint

Author: rnjtranjan

core/src/Streamly/Internal/Unicode/Stream.hs

@@ -190,8 +190,26 @@ encodeLatin1Lax = encodeLatin1
 -- UTF-8 decoding
 -------------------------------------------------------------------------------
 
+-- CodePoint represents a specific character in the Unicode standard. 
+-- The code point is a numerical value assigned to each character, 
+-- and UTF-8 encoding uses a variable number of bytes to represent 
+-- different code points.
+--
+-- Calculate the code point value: Depending on the type of the leading byte, 
+-- extract the significant bits from each byte of the sequence and combine them 
+-- to form the complete code point value. The specific bit manipulations will 
+-- differ based on the number of bytes used. 
 -- Int helps in cheaper conversion from Int to Char
 type CodePoint = Int
+
+-- DecodeState refers to the number of bytes remaining to complete the current
+-- UTF-8 character decoding. For ASCII characters (code points 0 to 127), 
+-- no decoding state is necessary because they are represented by a single byte. 
+-- Therefore, the decoding state for ASCII characters can be considered as 0.
+-- For multi-byte characters, the decoding state indicates the number of bytes 
+-- remaining to complete the character. It is usually initialized to a non-zero 
+-- value corresponding to the number of bytes in the multi-byte character, e.g 
+-- DecodeState will be 1 for 2-bytes char.
 type DecodeState = Word8
 
 -- We can divide the errors in three general categories:

core/src/Streamly/Unicode/Stream.hs

@@ -76,10 +76,10 @@
 module Streamly.Unicode.Stream
     (
       DecodeState
-    , DecodeError(..)
     , CodePoint
-    
-    -- * Construction (Decoding)
+    , DecodeError(..)
+
+    -- * Resumable UTF-8 decoding
     , decodeLatin1
     , decodeUtf8
     , decodeUtf8'