📝 Add record patch API docs

docs/api/set-target.md

@@ -0,0 +1,79 @@
+---
+name: set-target
+group: api
+category: patching
+update-time: 20260512
+description: Create a reusable record patch that rewrites the target field.
+key-word:
+    - patch
+    - target
+    - transform
+    - public
+---
+
+## Set-target
+
+Create a `RecordPatch` that rewrites `rec.target` to a fixed string. Use it when records should be reclassified or routed under a normalized target.
+
+### Interface
+
+```moonbit
+pub fn set_target(target : String) -> RecordPatch {}
+```
+
+#### input
+
+- `target : String` - Target value that should replace the original record target.
+
+#### output
+
+- `RecordPatch` - Patch that returns a record with the new target.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The patch copies the original record and replaces only its `target`.
+- Message, level, timestamp, and fields are preserved.
+- This helper is useful when several call sites should appear under one logical namespace.
+- It can be combined with `prefix_message(...)` or `append_fields(...)` in ordered pipelines.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Normalize Child Targets
+
+When several producers should share one target label:
+```moonbit
+let logger = Logger::new(console_sink(), target="worker")
+  .with_patch(set_target("worker.batch"))
+```
+
+In this example, downstream filters and sinks only see `worker.batch`.
+
+#### When Redirect Records Before Fanout
+
+When a routing layer expects one canonical target:
+```moonbit
+let patch = compose_patches([
+  set_target("audit"),
+  append_fields([field("source", "legacy")]),
+])
+```
+
+In this example, reclassification and enrichment happen in a predictable order.
+
+### Error Case
+
+e.g.:
+- If `target` is empty, the patch still rewrites the record target to an empty string.
+
+- If later patches also rewrite the target, the last applied patch wins.
+
+### Notes
+
+1. Use this helper for target normalization, not for filtering decisions.
+
+2. Prefer explicit patch order when multiple target rewrites are possible.
+