Be more careful when documenting the way that extensions are mapped to

Be more careful when documenting the way that extensions are mapped to
meta data. Note the effect of multiple extensions in all the Add*
directive descriptions, and explain that these directives add or override
existing mappings. Remove assumptions that there is only one extension.
Add note about how setting both handlers and media types could result in
unexpected results. Remove some highly misleading information such as that
the type is the extension left "after content-encoding and language
extensions have been removed"

PR: 3151

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@82403 13f79535-47bb-0310-9956-ffa450edef68