📝 Add async logger composition API docs

docs/api/async-logger-with-patch.md

@@ -0,0 +1,82 @@
+---
+name: async-logger-with-patch
+group: api
+category: async
+update-time: 20260512
+description: Attach record transformation logic to an async logger before records reach the queue.
+key-word:
+    - async
+    - logger
+    - patch
+    - public
+---
+
+## Async-logger-with-patch
+
+Attach a record patch to an async logger so each record is transformed before filtering and enqueue. This is the main API for async record rewriting without changing the sink implementation.
+
+### Interface
+
+```moonbit
+pub fn[S] AsyncLogger::with_patch(
+  self : AsyncLogger[S],
+  patch : @bitlogger.RecordPatch,
+) -> AsyncLogger[S] {}
+```
+
+#### input
+
+- `self : AsyncLogger[S]` - Base async logger to wrap.
+- `patch : RecordPatch` - Transformation applied to each record before enqueue.
+
+#### output
+
+- `AsyncLogger[S]` - A new async logger that rewrites each record before filtering and queue insertion.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Patch logic runs after record creation and field merging but before filtering and enqueue.
+- Existing patch logic is preserved and composed so the new patch wraps the current one.
+- The original async logger is not mutated.
+- Patching can normalize, redact, or enrich records before they consume queue capacity.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Async Record Enrichment
+
+When records should gain stable extra data before enqueue:
+```moonbit
+let logger = async_logger(console_sink())
+  .with_patch(@bitlogger.append_fields([
+    @bitlogger.field("channel", "async"),
+  ]))
+```
+
+In this example, the added fields are part of the record before filtering and queue insertion.
+
+#### When Need Redaction Before Queueing
+
+When sensitive fields should be removed early:
+```moonbit
+let logger = async_logger(console_sink())
+  .with_patch(@bitlogger.redact_fields(["token", "password"]))
+```
+
+In this example, sensitive values are rewritten before they enter the async pipeline.
+
+### Error Case
+
+e.g.:
+- If the patch removes or rewrites important data incorrectly, later filters and sinks will only see the patched version.
+
+- If callers need selection logic rather than transformation, `with_filter(...)` should be used instead.
+
+### Notes
+
+1. Use patches for transformation, not filtering decisions.
+
+2. Redaction before enqueue helps keep sensitive data out of the queued pipeline.