Improve documentation for specifying env vars

Summary: This syntax is somewhat obscure and awkward, try to document it more completely in the location users are most likely to run into it for the first time.

Test Plan: Generated documentation: https://secure.phabricator.com/file/view/PHID-FILE-ba958fd365e5312d9077/

Reviewers: codeblock, egillth, jungejason, tuomaspelkonen, aran

CC:

Differential Revision: 775

Author: epriestley

src/docs/configuration/upgrade_schema.diviner

@@ -15,7 +15,14 @@ To upgrade your database schema to the latest version, just run this command:
 
   PHABRICATOR_ENV=<your_config> path/to/phabricator/scripts/sql/upgrade_schema.php
 
-This will install all the patches that are new since you installed, or since the last time you ran this script.
+NOTE: Make sure you put 'PHABRICATOR_ENV=...' at the beginning of the line, not
+in the middle. The shell won't parse environmental variables declared after the
+command. You can also ##export PHABRICATOR_ENV=...## in your ~/.bashrc or
+~/.profile or similar, depending on which shell you use and how your system is
+configured.
+
+This will install all the patches that are new since you installed, or since the
+last time you ran this script.
 
 If your configuration uses an unprivileged user to connect to the database, you
 may have to override the default user so the schema changes can be applied with