Fix documents for ALLOWED_HOST_LIST, its default value differs between 1.15 and 1.16

[wxiaoguang]

As discussed in the backport PR

• Only allow webhook to send requests to allowed hosts (backport #17482) #17510

The newly added setting option ALLOWED_HOST_LIST for webhooks:

• In 1.15, the default should be *
• In 1.16 and later, the default should be external

This PR fixes the documents for 1.16 and later.

The original PR #17482 has been marked as breaking, so this PR can just skip-changelog

This PR should not be merged before the backport. If the backport is discarded, then this PR should also be discarded.

[codecov-commenter]

Codecov Report

Merging #17530 (1cb4407) into main (bd613c7) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #17530      +/-   ##
==========================================
+ Coverage   45.49%   45.50%   +0.01%     
==========================================
  Files         793      793              
  Lines       88811    88811              
==========================================
+ Hits        40401    40413      +12     
+ Misses      41897    41883      -14     
- Partials     6513     6515       +2     

Impacted Files	Coverage Δ
modules/git/repo_base_nogogit.go	82.85% <0.00%> (-2.86%)	⬇️
modules/git/utils.go	68.05% <0.00%> (-2.78%)	⬇️
modules/process/manager.go	72.83% <0.00%> (-2.47%)	⬇️
services/pull/pull.go	41.78% <0.00%> (-0.41%)	⬇️
models/issue_comment.go	51.60% <0.00%> (-0.30%)	⬇️
models/notification.go	66.15% <0.00%> (+0.87%)	⬆️
modules/notification/ui/ui.go	62.31% <0.00%> (+1.44%)	⬆️
modules/queue/queue_channel.go	96.66% <0.00%> (+1.66%)	⬆️
modules/notification/mail/mail.go	38.23% <0.00%> (+2.94%)	⬆️
modules/queue/workerpool.go	52.29% <0.00%> (+4.19%)	⬆️

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