Removed outdated limits documentation

[radarhere]

https://github.com/python-pillow/Pillow/blob/master/docs/reference/limits.rst

• Any call to internal python size functions for buffers or strings are currently returned as int32, not py_ssize_t. This limits the maximum buffer to 2GB for operations like frombytes and frombuffer.

• This also limits the size of buffers converted using a decoder. (decode.c:127)

#3791 replaced ints with py_ssize_t, so this is no longer correct.

For clarity, 'decode.c:127' was

Pillow/decode.c

Line 127 in 937199e

if (!PyArg_ParseTuple(args, PY_ARG_BYTES_LENGTH, &buffer, &bufsize))

at the time.