doc: Fix CHANGES entries to have .txt suffix and update READMEs

Although the wiki page for the new CHANGES and UPGRADE scheme
states that the files must have the ".txt" suffix, the READMEs
didn't.

Change-Id: I490306aa2cc24d6f014738e9ebbc78592efe0f05
(cherry picked from commit 7416703f)

doc/CHANGES-staging/README.md

@@ -9,10 +9,10 @@ it probably should. If there are multiple items that need documenting, you can
 add multiple files, each with their own description. If the message is going to
 be the same for each subject, then you can add multiple subject headers to one
 file. The "Subject: xxx" line is case sensitive! For example, if you are making
-a change to PJSIP, then you might add the file "res_pjsip_my_cool_feature" to
-this directory, with a short description of what it does. If you are adding
-multiple entries, they should be done in the same commit to avoid merge
-conflicts. Here's an example:
+a change to PJSIP, then you might add the file "res_pjsip_my_cool_feature.txt" to
+this directory, with a short description of what it does.  The files must  have
+the ".txt" suffix.  If you are adding multiple entries, they should be done in
+the same commit to avoid merge conflicts. Here's an example:
 
 > Subject: res_pjsip
 > Subject: Core

doc/CHANGES-staging/hide_messaging_ami_events.txt

+Subject: Messaging
+
+In order to reduce the amount of AMI and ARI events generated,
+the global "Message/ast_msg_queue" channel can be set to suppress
+it's normal channel housekeeping events such as "Newexten",
+"VarSet", etc. This can greatly reduce load on the manager
+and ARI applications when the Digium Phone Module for Asterisk
+is in use.  To enable, set "hide_messaging_ami_events" in
+asterisk.conf to "yes"  In Asterisk versions <18, the default
+is "no" preserving existing behavior.  Beginning with
+Asterisk 18, the option will default to "yes".

doc/CHANGES-staging/res_fax_negotiate_both

-Subject: res_fax
-
-Added configuration option "negotiate_both". This option is only used
-when a gateway is enabled, and a v21 preamble is detected. If this option
-is enabled, once a preamble is detected Asterisk will initiate negotiation
-requests to both T.38 enabled endpoint versus waiting, and forwarding a
-request from an initiating endpoint. Defaults to disabled.

doc/UPGRADE-staging/README.md

@@ -9,9 +9,10 @@ are multiple items that need documenting, you can add multiple files, each with
 their own description. If the message is going to be the same for each subject,
 then you can add multiple subject headers to one file. The "Subject: xxx" line
 is case sensitive! For example, if you are making a change to PJSIP, then you
-might add the file "res_pjsip_my_cool_feature" to this directory, with a short
-description of what it does. If you are adding multiple entries, they should be
-done in the same commit to avoid merge conflicts. Here's an example:
+might add the file "res_pjsip_my_cool_feature.txt" to this directory, with a
+short description of what it does. The files must have the ".txt" suffix.
+If you are adding multiple entries, they should be done in the same commit
+to avoid merge conflicts. Here's an example:
 
 > Subject: res_pjsip
 > Subject: Core