mod_proxy.xml: docs improvements

Tried to clarify the doubts raised in PR 61225
(related also to 60458) and also split a single note
into multiple ones to improve the reader's attention span
(reading through a long block of text may cause loss
of attention to important details, as it happened to me :)



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1828128 13f79535-47bb-0310-9956-ffa450edef68