docs: Update news, man page, and online help for restore --accumulate

As a side effect, reformat the NEWs entry for notmuch dump for
consistency with the notmuch restore NEWS submitted by Thomas
Schwinge.

NEWS

@@ -1,11 +1,20 @@
 Notmuch 0.10 (2011-xx-xx)
+=========================
 
-notmuch dump changes 
---------------------
+New command-line features
+-------------------------
 
-The dump command now takes an optional search term much like notmuch
-search/show/tag. The output file argument of dump is deprecated in
-favour of using stdout.
+Add "notmuch restore --accumulate" option
+
+  The --accumulate switch causes the union of the existing and new tags to be
+  applied, instead of replacing each message's tags as they are read in from
+  the dump file.
+
+Add search terms to  "notmuch dump"
+
+  The dump command now takes an optional search term much like notmuch
+  search/show/tag. The output file argument of dump is deprecated in
+  favour of using stdout.
 
 Notmuch 0.9 (2011-10-01)
 ========================

notmuch.1

@@ -480,7 +480,7 @@ section below for details of the supported syntax for <search-terms>.
 .RE
 
 .TP
-.BR restore " <filename>"
+.BR restore " [--accumulate] [<filename>]"
 
 Restores the tags from the given file (see
 .BR "notmuch dump" ")."
@@ -493,6 +493,10 @@ So if you've previously been using sup for mail, then the
 .B "notmuch restore"
 command provides you a way to import all of your tags (or labels as
 sup calls them).
+
+The --accumulate switch causes the union of the existing and new tags to be
+applied, instead of replacing each message's tags as they are read in from the
+dump file.
 .RE
 
 The

notmuch.c

@@ -393,13 +393,17 @@ static command_t commands[] = {
       "\tSee \"notmuch help search-terms\" for the search-term syntax.\n"      
  },
     { "restore", notmuch_restore_command,
-      "<filename>",
+      "[--accumulate] [<filename>]",
       "Restore the tags from the given dump file (see 'dump').",
+      "\tInput is read from the given filename, if any, or from stdin.\n"
       "\tNote: The dump file format is specifically chosen to be\n"
       "\tcompatible with the format of files produced by sup-dump.\n"
       "\tSo if you've previously been using sup for mail, then the\n"
       "\t\"notmuch restore\" command provides you a way to import\n"
-      "\tall of your tags (or labels as sup calls them)." },
+      "\tall of your tags (or labels as sup calls them).\n"
+      "\tThe --accumulate switch causes the union of the existing and new\n"
+      "\ttags to be applied, instead of replacing each message's tags as\n"
+      "\tthey are read in from the dump file."},
     { "config", notmuch_config_command,
       "[get|set] <section>.<item> [value ...]",
       "Get or set settings in the notmuch configuration file.",