Add documentation for the recently added folder-based searching.

Augmenting both the builtin help as well as the manual page.

notmuch.1

@@ -526,6 +526,8 @@ terms to match against specific portions of an email, (where
 
 	thread:<thread-id>
 
+	folder:<directory-path>
+
 The
 .B from:
 prefix is used to match the name or address of the sender of an email
@@ -569,6 +571,13 @@ internally by notmuch (and do not appear in email messages). These
 thread ID values can be seen in the first column of output from
 .B "notmuch search"
 
+The
+.B folder:
+prefix can be used to search for email message files that are
+contained within particular directories within the mail store. Only
+the directory components below the top-level mail database path are
+available to be searched.
+
 In addition to individual terms, multiple terms can be
 combined with Boolean operators (
 .BR and ", " or ", " not

notmuch.c

@@ -58,6 +58,7 @@ static const char search_terms_help[] =
     "\t\ttag:<tag> (or is:<tag>)\n"
     "\t\tid:<message-id>\n"
     "\t\tthread:<thread-id>\n"
+    "\t\tfolder:<directory-path>\n"
     "\n"
     "\tThe from: prefix is used to match the name or address of\n"
     "\tthe sender of an email message.\n"
@@ -82,6 +83,11 @@ static const char search_terms_help[] =
     "\tmessages). These thread ID values can be seen in the first\n"
     "\tcolumn of output from \"notmuch search\".\n"
     "\n"
+    "\tThe folder: prefix can be used to search for email message\n"
+    "\tfiles that are contained within particular directories within\n"
+    "\tthe mail store. Only the directory components below the top-level\n"
+    "\tmail database path are available to be searched.\n"
+    "\n"
     "\tIn addition to individual terms, multiple terms can be\n"
     "\tcombined with Boolean operators (\"and\", \"or\", \"not\", etc.).\n"
     "\tEach term in the query will be implicitly connected by a\n"