fully document :source_url_pattern (#1086)

lib/mix/tasks/docs.ex

@@ -124,9 +124,26 @@ defmodule Mix.Tasks.Docs do
     * `:source_ref` - The branch/commit/tag used for source link inference;
       default: "master".
 
-    * `:source_url_pattern` - Public URL of the project. Derived from
-      project's `:source_url` and `:source_ref`. Example:
-      "https://github.com/USER/APP/blob/master/%{path}#L%{line}"
+    * `:source_url_pattern` - Public URL of the project for source links. This is derived
+      automatically from the project's `:source_url` and `:source_ref` when using one of
+      the supported public hosting services (currently GitHub, GitLab, or Bitbucket). If
+      you are using one of those services with their default public hostname, you do not
+      need to set this configuration.
+
+      However, if using a different solution, or self-hosting, you will need to set this
+      configuration variable to a pattern for source code links. The value must be a string
+      of the full URI to use for links with the following variables available for interpolation:
+
+        * `%{path}`: the path of a file in the repo
+        * `%{line}`: the line number in the file
+
+      For GitLab/GitHub:
+
+          https://mydomain.org/user_or_team/repo_name/blob/master/%{path}#L%{line}"
+
+      For Bitbucket:
+
+          https://mydomain.org/user_or_team/repo_name/src/master/%{path}#cl-%{line}
 
     * `:output` - Output directory for the generated docs; default: "doc".
       May be overridden by command line argument.