Improve stale message

[sapk]

Some users are irritated by the stale bot.

We can maybe improve the message to help understand the goal and how it works.

[codecov-io]

Codecov Report

Merging #9920 into master will decrease coverage by 0.01%.
The diff coverage is 23.07%.

@@            Coverage Diff             @@
##           master    #9920      +/-   ##
==========================================
- Coverage   42.29%   42.28%   -0.02%     
==========================================
  Files         607      608       +1     
  Lines       79352    79392      +40     
==========================================
+ Hits        33561    33568       +7     
- Misses      41649    41681      +32     
- Partials     4142     4143       +1

Impacted Files	Coverage Δ
services/pull/commit_status.go	5.19% <0%> (-2.97%)	⬇️
services/gitdiff/gitdiff.go	66.36% <0%> (ø)	⬆️
routers/repo/pull.go	28.67% <0%> (-0.07%)	⬇️
models/commit_status.go	64.13% <100%> (-2.2%)	⬇️
routers/api/v1/repo/status.go	44.57% <50%> (ø)	⬆️
modules/structs/commit_status.go	54.54% <54.54%> (ø)
models/repo.go	49.76% <0%> (-0.14%)	⬇️
services/pull/patch.go	67.92% <0%> (ø)	⬆️
routers/repo/view.go	39.47% <0%> (+0.87%)	⬆️
... and 2 more

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[guillep2k]

Just a nit.