closes bpo-35848: Move all documentation regarding the readinto out of IOBase. (GH-11893)

Move all documentation regarding the readinto method into either io.RawIOBase or io.BufferedIOBase.

Corresponding changes to documentation in the _pyio.py module.
(cherry picked from commit 7b97ab35b2)

Co-authored-by: Steve Palmer <steve@srpalmer.me.uk>

Doc/library/io.rst

@@ -226,7 +226,7 @@ I/O Base Classes
    implementations represent a file that cannot be read, written or
    seeked.
 
-   Even though :class:`IOBase` does not declare :meth:`read`, :meth:`readinto`,
+   Even though :class:`IOBase` does not declare :meth:`read`
    or :meth:`write` because their signatures will vary, implementations and
    clients should consider those methods part of the interface.  Also,
    implementations may raise a :exc:`ValueError` (or :exc:`UnsupportedOperation`)
@@ -234,9 +234,7 @@ I/O Base Classes
 
    The basic type used for binary data read from or written to a file is
    :class:`bytes`.  Other :term:`bytes-like objects <bytes-like object>` are
-   accepted as method arguments too.  In some cases, such as
-   :meth:`~RawIOBase.readinto`, a writable object such as :class:`bytearray`
-   is required.  Text I/O classes work with :class:`str` data.
+   accepted as method arguments too.  Text I/O classes work with :class:`str` data.
 
    Note that calling any method (even inquiries) on a closed stream is
    undefined.  Implementations may raise :exc:`ValueError` in this case.
@@ -405,7 +403,8 @@ I/O Base Classes
 
       Read bytes into a pre-allocated, writable
       :term:`bytes-like object` *b*, and return the
-      number of bytes read.  If the object is in non-blocking mode and no bytes
+      number of bytes read.  For example, *b* might be a :class:`bytearray`.
+      If the object is in non-blocking mode and no bytes
       are available, ``None`` is returned.
 
    .. method:: write(b)
@@ -495,6 +494,7 @@ I/O Base Classes
 
       Read bytes into a pre-allocated, writable
       :term:`bytes-like object` *b* and return the number of bytes read.
+      For example, *b* might be a :class:`bytearray`.
 
       Like :meth:`read`, multiple reads may be issued to the underlying raw
       stream, unless the latter is interactive.
@@ -757,8 +757,7 @@ Text I/O
 .. class:: TextIOBase
 
    Base class for text streams.  This class provides a character and line based
-   interface to stream I/O.  There is no :meth:`readinto` method because
-   Python's character strings are immutable.  It inherits :class:`IOBase`.
+   interface to stream I/O.  It inherits :class:`IOBase`.
    There is no public constructor.
 
    :class:`TextIOBase` provides or overrides these data attributes and
@@ -1048,4 +1047,3 @@ The above implicitly extends to text files, since the :func:`open()` function
 will wrap a buffered object inside a :class:`TextIOWrapper`.  This includes
 standard streams and therefore affects the built-in function :func:`print()` as
 well.
-

Lib/_pyio.py

@@ -287,16 +287,15 @@ class IOBase(metaclass=abc.ABCMeta):
     derived classes can override selectively; the default implementations
     represent a file that cannot be read, written or seeked.
 
-    Even though IOBase does not declare read, readinto, or write because
+    Even though IOBase does not declare read or write because
     their signatures will vary, implementations and clients should
     consider those methods part of the interface. Also, implementations
     may raise UnsupportedOperation when operations they do not support are
     called.
 
     The basic type used for binary data read from or written to a file is
-    bytes. Other bytes-like objects are accepted as method arguments too. In
-    some cases (such as readinto), a writable object is required. Text I/O
-    classes work with str data.
+    bytes. Other bytes-like objects are accepted as method arguments too.
+    Text I/O classes work with str data.
 
     Note that calling any method (even inquiries) on a closed stream is
     undefined. Implementations may raise OSError in this case.
@@ -1759,8 +1758,7 @@ class TextIOBase(IOBase):
     """Base class for text I/O.
 
     This class provides a character and line based interface to stream
-    I/O. There is no readinto method because Python's character strings
-    are immutable. There is no public constructor.
+    I/O. There is no public constructor.
     """
 
     def read(self, size=-1):