Fix merge issues on stdout branch

The act of committing a PR that including part of these changes and then
reverting it meant that some of the changes from the original branch
didn't land in this new PR.   This commit adds the missing bits.

Lesson learned:  Create a new branch when submitting a PR on a
previously reverted PR

awx/api/templates/api/unified_job_stdout.md

@@ -12,6 +12,7 @@ Use the `format` query string parameter to specify the output format.
 * Plain Text: `?format=txt`
 * Plain Text with ANSI color codes: `?format=ansi`
 * JSON structure: `?format=json`
+* Downloaded Plain Text: `?format=txt_download`
 
 (_New in Ansible Tower 2.0.0_) When using the Browsable API, HTML and JSON
 formats, the `start_line` and `end_line` query string parameters can be used
@@ -20,4 +21,7 @@ to specify a range of line numbers to retrieve.
 Use `dark=1` or `dark=0` as a query string parameter to force or disable a
 dark background.
 
++Files over {{ settings.STDOUT_MAX_BYTES_DISPLAY|filesizeformat }} (configurable) will not display in the browser. Use the `txt_download`
++format to download the file directly to view it.
+
 {% include "api/_new_in_awx.md" %}