Issue #11726: Fix linecache example in the doc

Use a Python source file (linecache.__file__) instead of /etc/passwd.

Modify also linecache docstrings to clarify the linecache is written to cache
Python source files, not any text files.

Doc/library/linecache.rst

@@ -51,5 +51,5 @@ The :mod:`linecache` module defines the following functions:
 Example::
 
    >>> import linecache
-   >>> linecache.getline('/etc/passwd', 4)
-   'sys:x:3:3:sys:/dev:/bin/sh\n'
+   >>> linecache.getline(linecache.__file__, 8)
+   'import sys\n'

Lib/linecache.py

@@ -1,4 +1,4 @@
-"""Cache lines from files.
+"""Cache lines from Python source files.
 
 This is intended to read lines from modules imported -- hence if a filename
 is not found, it will look down the module search path for a file by
@@ -32,7 +32,7 @@ def clearcache():
 
 
 def getlines(filename, module_globals=None):
-    """Get the lines for a file from the cache.
+    """Get the lines for a Python source file from the cache.
     Update the cache if it doesn't contain an entry for this file already."""
 
     if filename in cache: