message: Update documentation for SavedModel tags.
diff: @@ -91,10 +91,17 @@ with an asset of the same name, only the first version is retained.
  #### Tags
  Each meta graph added to the SavedModel must be annotated with user specified
  -tags. The tags provide a means to identify the specific meta graph to load and
  -restore, along with the shared set of variables and assets. These tags
  -typically annotate a MetaGraph with its functionality (e.g. serving or
  -training), and possibly hardware specific aspects such as GPU.
  +tags, which reflect the meta graph capabilities or use-cases.
  +More specifically, these tags typically annotate a meta graph with its
  +functionality (e.g. serving or training), and possibly hardware specific aspects
  +such as GPU.
  +In the SavedModel, the meta graph def whose tag-set exactly matches those
  +specified in the loader API, will be the one loaded by the loader.
  +If no meta graph def is found matching the specified tags, an error is returned.
  +For example, a loader with a requirement to serve on GPU hardware would be able
  +to load only meta graph annotated with tags='serve,gpu' by specifying this set
  +of tags in tensorflow::LoadSavedModel(...).
  +
  #### Usage
  The typical usage of `builder` is as follows:
  