help: markup email addresses as inline literals

Otherwise we get useless links to [e-mail] in the HTML version
of the man pages.

help/environment.txt

@@ -50,7 +50,7 b' HGUSER'
     - hgrc files from the HGRCPATH
     - EMAIL
     - interactive prompt
-    - LOGNAME (with '@hostname' appended)
+    - LOGNAME (with ``@hostname`` appended)
 
     (deprecated, use .hgrc)
 

help/templates.txt

@@ -75,11 +75,11 b' List of filters:'
               the timezone: "Mon Sep 04 15:13:13 2006 0700".
 :domain:      Any text. Finds the first string that looks like an
               email address, and extracts just the domain
-              component. Example: 'User <[e-mail]>' becomes
-              'example.com'.
+              component. Example: ``User <[e-mail]>`` becomes
+              ``example.com``.
 :email:       Any text. Extracts the first string that looks like
-              an email address. Example: 'User <[e-mail]>'
-              becomes '[e-mail]'.
+              an email address. Example: ``User <[e-mail]>``
+              becomes ``[e-mail]``.
 :escape:      Any text. Replaces the special XML/XHTML characters
               "&", "<" and ">" with XML entities.
 :fill68:      Any text. Wraps the text to fit in 68 columns.