DOC: Add comments about UTF-16 source conversion

Author: Licht-T

pandas/_libs/parsers.pyx

@@ -686,6 +686,8 @@ cdef class TextReader:
                                  self.compression)
 
             if b'utf-16' in (self.encoding or b''):
+                # we need to read utf-16 through UTF8Recoder.
+                # if source is utf-16, convert source to utf-8 by UTF8Recoder.
                 source = com.UTF8Recoder(source, self.encoding.decode('utf-8'))
                 self.encoding = b'utf-8'
                 self.c_encoding = <char*> self.encoding

pandas/io/parsers.py

@@ -1673,6 +1673,7 @@ def __init__(self, src, **kwds):
 
         if kwds.get('compression') is None \
            and 'utf-16' in (kwds.get('encoding') or ''):
+            # if source is utf-16 plain text, convert source to utf-8
             if isinstance(src, compat.string_types):
                 src = open(src, 'rb')
                 self.handles.append(src)