mod_headers.xml: clarify the difference between

mod_headers.xml: clarify the difference between
                 onsuccess vs always

In PR 62380 a user was confused why Header set always
was not overriding a header set by a HTTP backend managed
via mod_proxy_http. The difference between 'onsuccess'
and 'always' is really subtle, even if somebody is familiar
with r->headers_out and r->err_headers_out and the httpd's
internals.

As Stefan mentioned over email, the absence of a "normalized"
headers list in the response should be explained, so I tried to
do so in this commit.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1844401 13f79535-47bb-0310-9956-ffa450edef68