📝 Add filter predicate API docs

docs/api/message-contains.md

@@ -0,0 +1,79 @@
+---
+name: message-contains
+group: api
+category: filtering
+update-time: 20260512
+description: Create a reusable record predicate that matches records by message fragment.
+key-word:
+    - message
+    - filter
+    - predicate
+    - public
+---
+
+## Message-contains
+
+Create a `RecordPredicate` that returns `true` when a log message contains the given fragment. This helper is mainly useful for diagnostics, temporary routing, and focused debugging.
+
+### Interface
+
+```moonbit
+pub fn message_contains(fragment : String) -> RecordPredicate {}
+```
+
+#### input
+
+- `fragment : String` - Substring expected to appear in `rec.message`.
+
+#### output
+
+- `RecordPredicate` - Predicate that matches records whose message contains the fragment.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Matching uses `String::contains(...)` on the full message text.
+- The predicate is case-sensitive unless the message was normalized earlier.
+- This helper is convenient for ad hoc triage when targets or fields are not enough.
+- It should usually complement more stable filters rather than replace them in long-term configs.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Watch A Temporary Error Pattern
+
+When a debugging session focuses on one phrase:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_filter(message_contains("retry failed"))
+```
+
+In this example, only messages carrying that fragment remain visible.
+
+#### When Combine With Target Filtering
+
+When the same message should be isolated inside one subsystem:
+```moonbit
+let predicate = all_of([
+  target_has_prefix("worker.sync"),
+  message_contains("timeout"),
+])
+```
+
+In this example, unrelated timeout messages from other targets are ignored.
+
+### Error Case
+
+e.g.:
+- If `fragment` is empty, the predicate effectively matches every message because every string contains an empty substring.
+
+- If messages are localized or reformatted frequently, fragment-based matching can become brittle.
+
+### Notes
+
+1. Prefer field- or target-based filtering for stable production rules.
+
+2. Message substring filters are most valuable during debugging and migration periods.
+