Explain what os.read() returns at end of file. This closes SF bug #550409.  Applying to release21-maint & release22-maint.
Explain what os.read() returns at end of file.
This closes SF bug #550409.  Applying to release21-maint & release22-maint.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator