diff --git a/docs/Gemfile.lock b/docs/Gemfile.lock
index 8d96e5d..934baab 100644
--- a/docs/Gemfile.lock
+++ b/docs/Gemfile.lock
@@ -208,7 +208,7 @@ GEM
       jekyll-seo-tag (~> 2.1)
     minitest (5.11.3)
     multipart-post (2.0.0)
-    nokogiri (1.10.1)
+    nokogiri (1.10.4)
       mini_portile2 (~> 2.4.0)
     octokit (4.13.0)
       sawyer (~> 0.8.0, >= 0.5.3)
diff --git a/docs/configuration/configuration.md b/docs/configuration/configuration.md
index 05cc299..6fca562 100644
--- a/docs/configuration/configuration.md
+++ b/docs/configuration/configuration.md
@@ -126,12 +126,14 @@ Multiple upstreams can either be configured by supplying a comma separated list
 
 Every command line argument can be specified as an environment variable by
 prefixing it with `OAUTH2_PROXY_`, capitalising it, and replacing hypens (`-`)
-with underscores (`_`). This is particularly useful for storing secrets outside
-of a configuration file or the command line.
+with underscores (`_`). If the argument can be specified multiple times, the
+environment variable should be plural (trailing `S`).
 
-For example, the `--cookie-secret` flag becomes `OAUTH2_PROXY_COOKIE_SECRET` and
-the `--set-authorization-header` flag becomes `OAUTH2_PROXY_SET_AUTHORIZATION_HEADER`.
+This is particularly useful for storing secrets outside of a configuration file
+or the command line.
 
+For example, the `--cookie-secret` flag becomes `OAUTH2_PROXY_COOKIE_SECRET`,
+and the `--email-domain` flag becomes `OAUTH2_PROXY_EMAIL_DOMAINS`.
 
 ## Logging Configuration