doc: fix platform notes hierarchy; don't use anydbm session db

Linux and Solaris are section headers, but Windows is not. Fix that
and move all the windows stuff under the Windows header.

Add a section for windows to not use anydbm as the session/otks store.
Window only has a dumb dbm written in Python available. gdbm/ndbm
compiled backends are not available. So recommend using sqlite or
redis for session db if they don't want to spin up mysql/postgresql.

Author: rouilj

doc/installation.txt

@@ -2124,10 +2124,28 @@ addresses, databases to back up, etc.
 Platform-Specific Notes
 =======================
 
+
+Windows
+-------
+
+.. index:: windows; choose non-default session/otk database
+
+Configure session database
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default the session and one time key (OTK) databases default to
+using anydbm as the session store. Because GNU dbm and Ndbm are
+available on Linux, this is a reasonable default. However Windows
+doesn't have compiled libraries for these databases. So it defaults to
+"dumb" dbm which is written in Python and is slow. So we recommend
+using `some other backend for your session database`_. SQLite or Redis
+can be used if you don't want to run a separate supported RDBMS
+database server.
+
 .. index:: windows; add Roundup to path
 
 Windows command-line tools
---------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 To make the command-line tools accessible in Windows, you need to update
 the "Path" environment variable in the Registry via a dialog box.
@@ -2151,7 +2169,7 @@ Panel" is directly accessible from "Start".
 I do not believe this is possible to do in previous versions of Windows.
 
 Use pip to install pyreadline3 for roundup-admin line editing
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 
 If you install pyreadline3_ using pip, roundup-admin will support
 command line editing and history.
@@ -2163,7 +2181,7 @@ This will remove the dreaded::
 warning when starting roundup-admin.
 
 Windows Server
---------------
+~~~~~~~~~~~~~~
 
 To have the Roundup web server start up when your machine boots up, there
 are two different methods, the scheduler and installing the service.
@@ -2173,7 +2191,7 @@ are two different methods, the scheduler and installing the service.
 .. _Using the Windows scheduler:
 
 1. Using the Windows scheduler
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+''''''''''''''''''''''''''''''
 
 Set up the following in Scheduled Tasks (note, the following is for a
 cygwin setup):
@@ -2205,7 +2223,7 @@ Schedule
 .. index:: windows; setup Roundup a service
 
 2. Installing the roundup server as a Windows service
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+'''''''''''''''''''''''''''''''''''''''''''''''''''''
 
 This is more Windows oriented and will make the Roundup server run as
 soon as the PC starts up without any need for a login or such. It will
@@ -2246,8 +2264,13 @@ If you are using Apache as the webserver you might want to use it with
 mod_python instead to serve out Roundup. In that case see the mod_python
 instructions above for details.
 
+Linux
+-----
+
+Make sure you read the instructions under `UNIX environment steps`_.
+
 Sendmail smrsh
---------------
+~~~~~~~~~~~~~~
 
 If you use Sendmail's ``smrsh`` mechanism, you will need to tell
 smrsh that roundup-mailgw is a valid/trusted mail handler
@@ -2262,11 +2285,6 @@ This is usually done via the following 2 steps:
 2. change your alias to ``"|roundup-mailgw <tracker_home>"``
 
 
-Linux
------
-
-Make sure you read the instructions under `UNIX environment steps`_.
-
 
 Solaris
 -------
@@ -2321,6 +2339,8 @@ the test.
 .. _`preventing spam`: customizing.html#preventing-spam
 .. _`Adding a new field to the classic schema`:
    customizing.html#adding-a-new-field-to-the-classic-schema
+.. _`some other backend for your session database`:
+   admin_guide.html#configuring-session-databases
 .. _`Tracking different types of issues`:
    customizing.html#tracking-different-types-of-issues
 .. _`customising roundup`: customizing.html