ARROW-13036: [Doc] Mention recommended file extension(s) for Arrow IPC

docs/source/format/Columnar.rst

@@ -1006,19 +1006,21 @@ message flatbuffer is read, you can then read the message body.
 
 The stream writer can signal end-of-stream (EOS) either by writing 8 bytes
 containing the 4-byte continuation indicator (``0xFFFFFFFF``) followed by 0
-metadata length (``0x00000000``) or closing the stream interface.
+metadata length (``0x00000000``) or closing the stream interface. We
+recommend the ".arrows" file extension for the streaming format although
+in many cases these streams will not ever be stored as files.
 
 IPC File Format
 ---------------
 
-We define a "file format" supporting random access that is build with
-the stream format. The file starts and ends with a magic string
-``ARROW1`` (plus padding). What follows in the file is identical to
-the stream format. At the end of the file, we write a *footer*
-containing a redundant copy of the schema (which is a part of the
-streaming format) plus memory offsets and sizes for each of the data
-blocks in the file. This enables random access any record batch in the
-file. See `File.fbs`_ for the precise details of the file footer.
+We define a "file format" supporting random access that is an extension of
+the stream format. The file starts and ends with a magic string ``ARROW1``
+(plus padding). What follows in the file is identical to the stream format.
+At the end of the file, we write a *footer* containing a redundant copy of
+the schema (which is a part of the streaming format) plus memory offsets and
+sizes for each of the data blocks in the file. This enables random access to
+any record batch in the file. See `File.fbs`_ for the precise details of the
+file footer.
 
 Schematically we have: ::
 
@@ -1034,8 +1036,9 @@ should be defined in a ``DictionaryBatch`` before they are used in a
 ``RecordBatch``, as long as the keys are defined somewhere in the
 file. Further more, it is invalid to have more than one **non-delta**
 dictionary batch per dictionary ID (i.e. dictionary replacement is not
-supported).  Delta dictionaries are applied in the order they appear in
-the file footer.
+supported). Delta dictionaries are applied in the order they appear in
+the file footer. We recommend the ".arrow" extension for files created with
+this format.
 
 Dictionary Messages
 -------------------