docs: issue2551311 document when index returned from roundup-server

in issue2551311 a user was confused when setting the tracker web to
the root URL path. While resolving this, it turns out there was no
explanation of what was returned when the root URL was accessed
compared to a tracker URL.

Added explanation that index is returned for multiple trackers and a
redirect for only one tracker.

Now that the man pages are available online, this is a reasonable
place to put the doc as there are no suitable places in the core docs.

Author: rouilj

share/man/man1/roundup-server.1

@@ -39,7 +39,9 @@ Have the server log using the Python logger with key roundup.http.
 \fB-i\fP \fIfile\fP
 Sets a filename to use as a template for generating the tracker index page.
 The variable "trackers" is available to the template and is a dict of all
-configured trackers.
+configured trackers. If you connect to the root of the server (without
+a tracker name), the default index page is shown. Using \fB-i\fP
+allows you to customize the index page.
 .TP
 \fB-I\fP \fIheader1[,header2,...]\fP
 Pass the header(s) and their values to the backend. This allow-list
@@ -98,11 +100,19 @@ the command-line. For convenience, you may edit the TRACKER_HOMES variable in
 the roundup-server file instead.  Make sure the name part doesn't include any
 url-unsafe characters like spaces, as these confuse the cookie handling in
 browsers like IE.
+.PP
+If you connect to the root directory (I.E. you do not include the
+\fBname\fP component in the URL, the server will display an index page
+of trackers served by roundup-server if more than one tracker is
+defined. If only one tracker is defined it will redirect to the tracker.
 .SH EXAMPLES
 .TP
 .B roundup-server -p 9000 bugs=/var/tracker reqs=/home/roundup/group1
 Start the server on port \fB9000\fP serving two trackers; one under
 \fB/bugs\fP and one under \fB/reqs\fP.
+.PP
+If you connect to the server at the root level (http://host:9000/) it
+will display a list of the available trackers.
 
 .SH CONFIGURATION FILE
 See the "admin_guide" in the Roundup "doc" directory.