Document new string interpolation behavior

docs/bound-loggers.md

@@ -82,7 +82,7 @@ Now `log` is a *bound logger* of type {class}`~structlog.typing.FilteringBoundLo
 
 Now if you call `log.info("Hello, %s!", "world", number=42)` the following happens:
 
-1. `"world"` gets interpolated into `"Hello, %s!"`, making the event "Hello, world!".
+1. `"world"` gets interpolated into `"Hello, %s!"`, making the event "Hello, world!"[^interpolation].
 2. The *bound logger*'s context gets copied and the key-value pairs from the `info` call are added to it.
    It becomes an *event dict* and is `{"foo": "bar", "number": 42}` now.
 3. The event from step 1 is added too.
@@ -99,6 +99,7 @@ Now if you call `log.info("Hello, %s!", "world", number=42)` the following happe
    By replacing the last processor, you decide on the **format** of your logs.
    For example, if you wanted JSON logs, you just have to replace the last processor with {class}`structlog.processors.JSONRenderer`.
 
+[^interpolation]: String interpolation only takes place if you pass positional arguments.
 
 (filtering)=
 

src/structlog/typing.py

@@ -155,6 +155,9 @@ class FilteringBoundLogger(BindableLogger, Protocol):
        String interpolation using positional arguments.
     .. versionadded:: 22.2.0
        Async variants ``alog()``, ``adebug()``, ``ainfo()``, and so forth.
+    .. versionchanged:: 22.3.0
+       String interpolation is only attempted if positional arguments are
+       passed.
     """
 
     def bind(self, **new_values: Any) -> FilteringBoundLogger: