docs: reference PhysicalOptimizerRuleExportable in SessionContext docstring

Point the `physical_optimizer_rules` argument docs at the new
`PhysicalOptimizerRuleExportable` Protocol instead of describing the duck type inline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/datafusion/context.py

@@ -546,8 +546,9 @@ def __init__(
             config: Session configuration options.
             runtime: Runtime configuration options.
             physical_optimizer_rules: User-defined physical optimizer rules to
-                append to the default set. Each item is an object that exposes a
-                ``__datafusion_physical_optimizer_rule__`` method returning a
+                append to the default set. Each item is a
+                :class:`PhysicalOptimizerRuleExportable` — an object exposing a
+                ``__datafusion_physical_optimizer_rule__`` method that returns a
                 PyCapsule around a ``FFI_PhysicalOptimizerRule`` (typically built
                 in a separate compiled extension). DataFusion provides no FFI
                 bridge for logical optimizer or analyzer rules, and there is no