Update comments in FileIO to match current code

Author: cmaloney

Modules/_io/fileio.c

@@ -725,8 +725,11 @@ _io.FileIO.readall
 
 Read all data from the file, returned as bytes.
 
-In non-blocking mode, returns as much as is immediately available,
-or None if no data is available.  Return an empty bytes object at EOF.
+Reads until either there is an error or read() returns size 0 (indicates EOF).
+If the file is already at EOF returns empty bytes.
+
+In non-blocking mode, returns as much data as could be read before EAGAIN. If no
+data is available (EAGAIN is returned before bytes are read) returns None.
 [clinic start generated code]*/
 
 static PyObject *
@@ -846,9 +849,14 @@ _io.FileIO.read
 
 Read at most size bytes, returned as bytes.
 
-Only makes one system call, so less data may be returned than requested.
-In non-blocking mode, returns None if no data is available.
-Return an empty bytes object at EOF.
+If size is less than 0, read all bytes in the file making multiple read calls.
+See ``_io.FileIO.readall``.
+
+Attempts to make only one system call, retrying only per PEP-475 (EINTR). This
+means less data may be returned than requested.
+
+In non-blocking mode, returns None if no data is available. Return an empty
+bytes object at EOF.
 [clinic start generated code]*/
 
 static PyObject *