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c, e, m and x flags are specific to the GNU libc. Python 2 does basically pass the mode to fopen() unmodified (there is one exception, the U flag). fopen() of Visual C++ 2005 has other flags:
c
Enable the commit flag for the associated filename so that the contents of the file buffer are written directly to disk if either fflush or _flushall is called.
n
Reset the commit flag for the associated filename to "no-commit." This is the default. It also overrides the global commit flag if you link your program with COMMODE.OBJ. The global commit flag default is "no-commit" unless you explicitly link your program with COMMODE.OBJ (see Link Options).
N
Specifies that the file is not inherited by child processes.
S
Specifies that caching is optimized for, but not restricted to, sequential access from disk.
R
Specifies that caching is optimized for, but not restricted to, random access from disk.
T
Specifies a file as temporary. If possible, it is not flushed to disk.
D
Specifies a file as temporary. It is deleted when the last file pointer is closed.
ccs=ENCODING
Specifies the coded character set to use (UTF-8, UTF-16LE, or UNICODE) for this file. Leave unspecified if you want ANSI encoding. This option is available in Visual C++ 2005 and later.
http://msdn.microsoft.com/en-us/library/yeby3zcb(v=vs.80).aspx
I don't think that non standard modes should be documented in Python doc, but we may add links to specific documentations like the GNU libc and Microsoft fopen().
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This issue is specific to Python 2, Python 3 doesn't use fopen() anymore.