Updated documentation of go_prefix flag (#1046)

* Update README.rst

* go_prefix flag is equivalent to prefix directive

* include root build file phrase

* fix gazelle rule formatting

README.rst

@@ -381,8 +381,9 @@ The following flags are accepted:
 | :flag:`-go_prefix example.com/repo`                          |                                        |
 +--------------------------------------------------------------+----------------------------------------+
 | A prefix of import paths for libraries in the repository that corresponds to                          |
-| the repository root. Gazelle infers this from the ``go_prefix`` rule in the                           |
-| root BUILD.bazel file, if it exists. If not, this option is mandatory.                                |
+| the repository root. Equivalent to setting the ``# gazelle:prefix`` directive                         |
+| in the root BUILD.bazel file or the ``prefix`` attribute of the ``gazelle`` rule. If                  |
+| neither of those are set, this option is mandatory.                                                   |
 |                                                                                                       |
 | This prefix is used to determine whether an import path refers to a library                           |
 | in the current repository or an external dependency.                                                  |