docs: improve unsafe_reentrant warning in docstrings

Address PR feedback:
- Add explicit warning about unsafe_reentrant making no guarantees
- Remove parenthetical about restoring pre-lock behavior
- Update both type definition and Agent.__init__ docstrings for consistency

Author: strands-agent

src/strands/agent/agent.py

@@ -189,8 +189,9 @@ def __init__(
                 Implement a custom HookProvider for custom retry logic, or pass None to disable retries.
             concurrent_invocation_mode: Mode controlling concurrent invocation behavior.
                 Defaults to "throw" which raises ConcurrencyException if concurrent invocation is attempted.
-                Set to "unsafe_reentrant" to skip lock acquisition entirely, allowing concurrent invocations
-                (restores pre-locking behavior, use with caution).
+                Set to "unsafe_reentrant" to skip lock acquisition entirely, allowing concurrent invocations.
+                Warning: "unsafe_reentrant" makes no guarantees about resulting behavior and is provided
+                only for advanced use cases where the caller understands the risks.
 
         Raises:
             ValueError: If agent id contains path separators.

src/strands/types/agent.py

@@ -15,5 +15,9 @@
 
 Values:
     throw: Raises ConcurrencyException if concurrent invocation is attempted (default).
-    unsafe_reentrant: Allows concurrent invocations without locking (unsafe, restores pre-lock behavior).
+    unsafe_reentrant: Allows concurrent invocations without locking.
+
+Warning:
+    The ``unsafe_reentrant`` mode makes no guarantees about resulting behavior and is
+    provided only for advanced use cases where the caller understands the risks.
 """