[exim-dev] documentation tweak

some proposed changes in the VERP section

http://www.exim.org/exim-html-4.62/doc/html/spec_html/index.html#toc0412

first, change the example code from:

verp_smtp:
   driver = smtp
   max_rcpt = 1
   return_path = \
     ${if match {$return_path}{^(.+?)-request@???\$}\
       {$1-request=$local_part%$domain@???}fail}

to this:

verp_smtp:
   driver = smtp
   max_rcpt = 1
   return_path = \
     ${if match {$return_path}{^(.+?)-request@???\$}\
       {$1-request+$local_part=$domain@???}fail}

rationale: the current example uses a % sign, and many internet
mailers are configured to complain about % in local parts and
reject inbound mail when it appears in a local part anywhere.
this alternative works in most places.

second, add mention of local_part_suffix and
local_part_suffix_optional as methods of fielding
returned email.

richard