[3.12] Doc: Improve description of GET_LEN opcode (pythonGH-114583) (python#125103)

Doc: Improve description of ``GET_LEN`` opcode (pythonGH-114583)
(cherry picked from commit e8773e5)

Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>

Author: miss-islington

Doc/library/dis.rst

@@ -850,7 +850,8 @@ iterations of the loop.
 
 .. opcode:: GET_LEN
 
-   Perform ``STACK.append(len(STACK[-1]))``.
+   Perform ``STACK.append(len(STACK[-1]))``. Used in :keyword:`match` statements where
+   comparison with structure of pattern is needed.
 
    .. versionadded:: 3.10