Issue #11726: clarify linecache doc: linecache is written to cache Python

source files, even if "it works" with other text files encoded to UTF-8.

Doc/library/linecache.rst

@@ -9,11 +9,15 @@
 
 --------------
 
-The :mod:`linecache` module allows one to get any line from any file, while
+The :mod:`linecache` module allows one to get any line from a Python source file, while
 attempting to optimize internally, using a cache, the common case where many
 lines are read from a single file.  This is used by the :mod:`traceback` module
 to retrieve source lines for inclusion in  the formatted traceback.
 
+The :func:`tokenize.open` function is used to open Python source files. This
+function uses :func:`tokenize.detect_encoding` to get the encoding of the
+Python source file.
+
 The :mod:`linecache` module defines the following functions:
 
 
@@ -49,4 +53,3 @@ Example::
    >>> import linecache
    >>> linecache.getline('/etc/passwd', 4)
    'sys:x:3:3:sys:/dev:/bin/sh\n'
-