Add comments explaining reason behind $http_host_with_default

+ add CHANGELOG.md entry

CHANGELOG.md

@@ -27,6 +27,7 @@ are not the same (O Schwede) c4e83c5
 - Remove deprecated git-annex configuration 527b942
 - Expose GitLab Workhorse configuration file 835144e
 - EE: Add a tracking database for GitLab Geo f1077d10
+- Provide default Host header for requests that do not have one
 8.17.3

doc/settings/nginx.md

@@ -498,8 +498,10 @@ server {
       return 418;
   }
+  # For protocol upgrades from HTTP/1.0 to HTTP/1.1 we need to provide Host header if its missing
   if ($http_host = "") {
-    set $http_host_with_default "localhost";
+  # use one of values defined in server_name
+    set $http_host_with_default "git.example.com";
   }
   if ($http_host != "") {