documentation update to clarify CLI option (#491)

* documentation update to clarify CLI option

- related to #490, updates the
  documentation to identify that --checkout-path requires an absolute
  oath, not a relative path.

* Update Sources/docc/DocCDocumentation.docc/distributing-documentation-to-other-developers.md

Author: heckj

Sources/docc/DocCDocumentation.docc/distributing-documentation-to-other-developers.md

@@ -61,25 +61,28 @@ service configuration flags like so:
 docc convert […] \
     --source-service github \
     --source-service-base-url https://github.com/<org>/<repo>/blob/<branch> \
-    --checkout-path <path to local checkout>
+    --checkout-path <absolute path to local checkout>
 ```
 
 **GitLab**
 ```bash
 docc convert […] \
     --source-service gitlab \
     --source-service-base-url https://gitlab.com/<org>/<repo>/-/tree/<branch> \
-    --checkout-path <path to local checkout>
+    --checkout-path <absolute path to local checkout>
 ```
 
 **BitBucket**
 ```bash
 docc convert […] \
     --source-service bitbucket \
     --source-service-base-url https://bitbucket.org/<org>/<repo>/src/<branch> \
-    --checkout-path <path to local checkout>
+    --checkout-path <absolute path to local checkout>
 ```
 
+> Note: The option `--checkout-path` expects an absolute path to where the package 
+is checked out, not a relative path.
+
 These arguments can also be provided to `swift package generate-documentation`
 if you're using the SwiftPM DocC Plugin or via the `OTHER_DOCC_FLAGS` build
 setting when building in Xcode.