📝 Add record patch API docs

docs/api/redact-fields.md

@@ -0,0 +1,80 @@
+---
+name: redact-fields
+group: api
+category: patching
+update-time: 20260512
+description: Create a reusable record patch that redacts multiple field keys.
+key-word:
+    - patch
+    - redact
+    - fields
+    - public
+---
+
+## Redact-fields
+
+Create a `RecordPatch` that replaces the value of every field whose key appears in a provided key list. Use it for shared masking rules across several sensitive fields.
+
+### Interface
+
+```moonbit
+pub fn redact_fields(keys : Array[String], placeholder~ : String = "***") -> RecordPatch {}
+```
+
+#### input
+
+- `keys : Array[String]` - Field keys whose values should be redacted.
+- `placeholder : String` - Replacement value written into each matching field.
+
+#### output
+
+- `RecordPatch` - Patch that returns a record with matching field values redacted.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The patch maps over the record field list and rewrites any field whose key is contained in `keys`.
+- Non-matching fields are preserved.
+- The same placeholder is applied to every matching key in the set.
+- This helper is useful for masking bundles such as `token`, `password`, and `secret` with one patch.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Mask Several Sensitive Keys
+
+When one logger must protect multiple credentials:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_patch(redact_fields(["token", "password", "secret"]))
+```
+
+In this example, every matching field value is masked before the sink sees it.
+
+#### When Share One Custom Mask
+
+When the output should show one explicit policy marker:
+```moonbit
+let patch = redact_fields([
+  "access_key",
+  "session_key",
+], placeholder="[hidden]")
+```
+
+In this example, all matching fields use the same replacement text.
+
+### Error Case
+
+e.g.:
+- If `keys` is empty, the patch behaves like a pass-through field mapper.
+
+- If the same key appears multiple times in `keys`, matching behavior is unchanged because membership is what matters.
+
+### Notes
+
+1. This helper is better than stacking many single-key patches when all keys share the same placeholder policy.
+
+2. It only rewrites structured fields, not text inside the message body.
+