dev-manual: bblock: use warning block instead of attention

Sphinx recommends using warning and note here:
https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#directives

Suggested-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 7d5eb0cee5b2b7096969819d7d7ce569a3c92f27)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/bblock.rst

@@ -19,7 +19,7 @@ recipes that depend on it.
 To prevent this, use ``bblock`` to lock specific tasks or recipes to
 specific signatures, forcing BitBake to use the sstate cache for them.
 
-.. attention::
+.. warning::
 
    Use ``bblock`` only during the development phase.