docs: Include ALU opcode descriptions

If we have a description for an ALU opcode in NIR, include it with our generated
HTML documentation so people don't need to go to nir_opcodes.py anyway because
the documentation is missing the documentation ;-)

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Jesse Natalie <jenatali@microsoft.com>
Tested-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22929>

docs/_exts/nir.py

@@ -58,6 +58,8 @@ def to_yn(b):
      - ${to_yn('associative' in op.algebraic_properties)}
      - ${to_yn('2src_commutative' in op.algebraic_properties)}
 
+${("**Description:** " + op.description) if op.description != "" else ""}
+
 **Constant-folding:**
 
 .. code-block:: c