fix: update docs for metadata external (#126)

docs/user-guide/configuration/metadata.md

@@ -15,13 +15,22 @@ toc:
 
 ## What is Metadata?
 
-Metadata is a structured key/value storage of relevant information about a [build](../../about/appendix/domain#build). Metadata will be shared with subsequent builds in the same [workflow](../../about/appendix/domain#workflow). It can be updated or retrieved throughout the build by using the built-in [meta CLI](https://github.com/screwdriver-cd/meta-cli) in the [steps](../../about/appendix/domain#step).
+Metadata is a structured key/value storage of relevant information about a [build](../../about/appendix/domain#build). It can be updated or retrieved throughout the build by using the built-in [meta CLI](https://github.com/screwdriver-cd/meta-cli) in the [steps](../../about/appendix/domain#step).
 
 ## Manipulating Metadata
 
 Screwdriver provides the shell command `meta get` to extract information from the meta store and `meta set` to save information to the meta store.
 
-Example:
+### Same pipeline
+
+Screwdriver build can retrieve metadata set by itself or by the previous builds within the same pipeline.
+
+Example: `build1` -> `build2` -> `build3`
+
+`build2`'s metadata will consist of metadata set by itself and `build1`
+
+`build3`'s metadata will consist of metadata from `build2` (which also includes metadata from `build1`)
+
 ```bash
 $ meta set example.coverage 99.95
 $ meta get example.coverage
@@ -36,3 +45,15 @@ $ meta set foo[2].bar[1] baz
 $ meta get foo
 [null,null,{"bar":[null,"baz"]}]
 ```
+
+### External pipeline
+
+Screwdriver build can also access metadata set by an external job which triggers it.
+
+Example: `sd@123:publish` -> `build1`. Then inside `build1`:
+```
+$ meta get example --external sd@123:publish
+{"coverage":99.95}
+```
+
+Notes: `meta set` is not allowed for external builds.