Need to update documentation about rfc56: Tighten control of access to mapfiles and templates

[nsavard]

The documentation has to be updated based on rfc56: html and pdf. The "MAP" tag in the mapfile, the "SYMBOLSET/END" tag in the symbol file, and the magic word in the query template have to be described.

[dmorissette]

One of the key issues here is that the SYMBOLSET/END tags do not appear to have ever been documented anywhere (or I could not find them anyway), so the symbolset docs should be updated not only in light of RFC-56, but also to indicate that in previous releases the SYMBOLSET/END tags existed and were optional.

[havatv]

Fixed for 6.0 and trunk documentation in r12037 (added notes about MAP and SYMBOLSET + included a link to the template document in layer-expression). The Apache environment variables are currently only documented in the RFC. I added a note on that in #3599. Closing.