Make it clearer that the template dir is relative to synapse's root dir (#5543)

Helps address #5444

changelog.d/5543.misc

+Make the config clearer in that email.template_dir is relative to the Synapse's root directory, not the `synapse/` folder within it.

docs/sample_config.yaml

@@ -1070,11 +1070,13 @@ password_config:
 #   app_name: Matrix
 #
 #   # Enable email notifications by default
+#   #
 #   notif_for_new_users: True
 #
 #   # Defining a custom URL for Riot is only needed if email notifications
 #   # should contain links to a self-hosted installation of Riot; when set
 #   # the "app_name" setting is ignored
+#   #
 #   riot_base_url: "http://localhost/riot"
 #
 #   # Enable sending password reset emails via the configured, trusted
@@ -1087,16 +1089,22 @@ password_config:
 #   #
 #   # If this option is set to false and SMTP options have not been
 #   # configured, resetting user passwords via email will be disabled
+#   #
 #   #trust_identity_server_for_password_resets: false
 #
 #   # Configure the time that a validation email or text message code
 #   # will expire after sending
 #   #
 #   # This is currently used for password resets
+#   #
 #   #validation_token_lifetime: 1h
 #
 #   # Template directory. All template files should be stored within this
-#   # directory
+#   # directory. If not set, default templates from within the Synapse
+#   # package will be used
+#   #
+#   # For the list of default templates, please see
+#   # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
 #   #
 #   #template_dir: res/templates
 #

synapse/config/emailconfig.py

@@ -233,11 +233,13 @@ class EmailConfig(Config):
         #   app_name: Matrix
         #
         #   # Enable email notifications by default
+        #   #
         #   notif_for_new_users: True
         #
         #   # Defining a custom URL for Riot is only needed if email notifications
         #   # should contain links to a self-hosted installation of Riot; when set
         #   # the "app_name" setting is ignored
+        #   #
         #   riot_base_url: "http://localhost/riot"
         #
         #   # Enable sending password reset emails via the configured, trusted
@@ -250,16 +252,22 @@ class EmailConfig(Config):
         #   #
         #   # If this option is set to false and SMTP options have not been
         #   # configured, resetting user passwords via email will be disabled
+        #   #
         #   #trust_identity_server_for_password_resets: false
         #
         #   # Configure the time that a validation email or text message code
         #   # will expire after sending
         #   #
         #   # This is currently used for password resets
+        #   #
         #   #validation_token_lifetime: 1h
         #
         #   # Template directory. All template files should be stored within this
-        #   # directory
+        #   # directory. If not set, default templates from within the Synapse
+        #   # package will be used
+        #   #
+        #   # For the list of default templates, please see
+        #   # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
         #   #
         #   #template_dir: res/templates
         #