Update pam documentation (#6518)

* document pam email extraction

* remove superfluous newline

.env.production.sample

@@ -141,8 +141,12 @@ STREAMING_CLUSTER_NUM=1
 # GID=1000
 
 # PAM authentication (optional)
+# PAM authentication uses for the email generation the "email" pam variable
+# and optional as fallback PAM_DEFAULT_SUFFIX
+# The pam environment variable "email" is provided by:
+# https://github.com/devkral/pam_email_extractor
 # PAM_ENABLED=true
-# Suffix for email address generation (nil by default)
+# Fallback Suffix for email address generation (nil by default)
 # PAM_DEFAULT_SUFFIX=pam
 # Name of the pam service (pam "auth" section is evaluated)
 # PAM_DEFAULT_SERVICE=rpam