docs/uio: Document StringIO/BytesIO(alloc_size) constructors.
@@ -112,3 +112,18 @@ Classes
.. method:: getvalue()
Get the current contents of the underlying buffer which holds data.
+
+.. class:: StringIO(alloc_size)
+.. class:: BytesIO(alloc_size)
+
+ Create an empty `StringIO`/`BytesIO` object, preallocated to hold up
+ to *alloc_size* number of bytes. That means that writing that amount
+ of bytes won't lead to reallocation of the buffer, and thus won't hit
+ out-of-memory situation or lead to memory fragmentation. These constructors
+ are a MicroPython extension and are recommended for usage only in special
+ cases and in system-level libraries, not for end-user applications.
+
+ .. admonition:: Difference to CPython
+ :class: attention
+
+ These constructors are a MicroPython extension.
