DOC: Clarify %HP description in log-format

DOC: Clarify %HP description in log-format

%HP is used to report HTTP request URI in logs, which might be relative
or absolute. Description in documentation should not suggest that it
behaves exactly the same as "path" sample fetch.

This is even more important after 30ee1efe676e8264af16bab833c621d60a72a4d7
because right now, when HTTP2 is a standard, %HP usually returns absolute
URI.

This might be backported as far as 2.1

(cherry picked from commit 21acc33266cc04eac5e70c22839d622284a6f46a)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>