docs: improve godoc and documentation [skip ci]

pkg/modules/pdfengines/README.md

@@ -1,14 +1,32 @@
 # Adding PDF Engine Features
 
-When adding a new PDF engine capability (e.g., bookmarks, watermark, stamp, embed), you must update the Makefile to include the corresponding engine list variable and flag. Every `--pdfengines-*-engines` flag registered in `pkg/modules/pdfengines/pdfengines.go` must have a matching entry in the Makefile:
+Each new PDF engine capability (e.g., bookmarks, watermark, stamp, embed) requires a matching Makefile entry. The Makefile variables control which engines are passed to Gotenberg at `make run` and `make test-integration` time (via `compose.yaml`). If you skip this step, the flag still works when set manually, but `make run` falls back to the default defined in `pdfengines.go`, which may not include the new engine.
 
-1. **Add a variable** in the Makefile's variable block (around line 60-70):
+Every `--pdfengines-*-engines` flag registered in `pkg/modules/pdfengines/pdfengines.go` must have a corresponding variable and flag in the Makefile:
+
+1. **Add a variable** in the Makefile's variable block (around line 60 to 70):
    ```makefile
    PDFENGINES_<FEATURE>_ENGINES=<default engines>
    ```
-2. **Add the flag** in the Makefile's command args block (around line 140-155):
-   ```makefile
-   --pdfengines-<feature>-engines=$(PDFENGINES_<FEATURE>_ENGINES) \
+2. **Add the flag** in `compose.yaml`'s command args:
+   ```yaml
+   - "--pdfengines-<feature>-engines=${PDFENGINES_<FEATURE>_ENGINES}"
    ```
 
-The default value should match what is defined in `pdfengines.go`'s `fs.StringSlice(...)` call for that flag.
+The default value must match the `fs.StringSlice(...)` call for that flag in `pdfengines.go`.
+
+## Example: Rotate
+
+The rotate feature was added with two engines (`pdfcpu` and `pdftk`). Here is what the additions look like:
+
+**Makefile** (variable block):
+
+```makefile
+PDFENGINES_ROTATE_ENGINES=pdfcpu,pdftk
+```
+
+**compose.yaml** (command args):
+
+```yaml
+- "--pdfengines-rotate-engines=${PDFENGINES_ROTATE_ENGINES}"
+```

pkg/modules/pdfengines/doc.go

@@ -1,3 +1,3 @@
-// Package pdfengines a way to gather and manage multiple modules that
-// implement the gotenberg.PdfEngine interface.
+// Package pdfengines gathers and manages modules that implement
+// gotenberg.PdfEngine.
 package pdfengines