📝 Add record patch API docs

docs/api/redact-field.md

@@ -0,0 +1,77 @@
+---
+name: redact-field
+group: api
+category: patching
+update-time: 20260512
+description: Create a reusable record patch that redacts one field key.
+key-word:
+    - patch
+    - redact
+    - fields
+    - public
+---
+
+## Redact-field
+
+Create a `RecordPatch` that replaces the value of every field whose key matches the given key. Use it when one sensitive field must be masked before records reach sinks.
+
+### Interface
+
+```moonbit
+pub fn redact_field(key : String, placeholder~ : String = "***") -> RecordPatch {}
+```
+
+#### input
+
+- `key : String` - Field key whose matching values should be replaced.
+- `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 full field list and rewrites each field whose key equals `key`.
+- Non-matching fields are preserved unchanged.
+- All matching entries are redacted, not just the first one.
+- The default placeholder is `"***"`, but callers can provide a different mask string.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Mask One Sensitive Field
+
+When authentication logs may carry a token:
+```moonbit
+let logger = Logger::new(console_sink(), target="auth")
+  .with_patch(redact_field("token"))
+```
+
+In this example, every `token` field value is replaced before output.
+
+#### When Use A Custom Placeholder
+
+When compliance rules require a specific visible marker:
+```moonbit
+let patch = redact_field("password", placeholder="[redacted]")
+```
+
+In this example, matching field values become `[redacted]` instead of the default mask.
+
+### Error Case
+
+e.g.:
+- If the record has no matching field key, the patch returns a structurally identical field list.
+
+- If `placeholder` is empty, matching values are replaced by an empty string.
+
+### Notes
+
+1. This helper only rewrites fields; it does not search message text.
+
+2. Use `redact_fields(...)` when several keys should share the same masking rule.
+