Improve SQL Server pyodbc documentation

Improve SQL Server pyodbc documentation

While we were told years ago that ODBC is intended to be used with
DSNs only, however this use does not correspond well with how most
other database connectivity systems work in that modern systems
already have their own registries of connection information in any
case, meaning this is usually the best place to add details such
as hostnames and driver names, rather than having them locked away
in a server-specific ODBC registry.    So here we dial back the
language that one style or another of connecting is "preferred";
both styles are supported equally, and the critical advantage of
hostname mapping in that the target database name is both explicit
as well as modifyable is also added.

Add additional background for how DSNs work and refine other
sentences.   "URL encoding" is the correct terminology for
adding spaces and special characters to a URL.

Change-Id: I13a74432976e6d3166633b98f9bb84c4856caac8