RFD: MFC hold time guidelines

https://reviews.freebsd.org/D35405 brought to light the lack of 
documented MFC hold time guidelines for src (only repo that uses that) 
beyond: barring critical fixes authorized by release engineering or the 
security officer, 3 days is the minimum. I'd like to have general 
guidelines hashed out and added to the committer's guide. To get things 
started, here's what Ed Maste reported using:

instant MFC: security or critical build fixes, or other critical 
changes, with coordination/approval of re@ or so@
3 days: straightforward bug fixes or minor improvements where there is a 
(presumed) very low probability of introducing a regression. e.g. typo 
fixes, man page updates
1 week: default timeout for most changes with low-medium presumed 
probability of introducing regressions e.g. adding to a driver's 
supported devices list, general bug fixes and new features
2 weeks, 3 weeks, 1 month: longer timeouts for changes with increasing 
likelihood of environment-dependent bugs or unique or rare corner cases 
e.g. major updates to contrib software, significant rework to kernel 
subsystems
Looking at my commit history over the last several years "1 week" is 
most common. I used "3 days" and "2 weeks" each about 1/3 as frequently 
as "1 week." "1 month" and "3 weeks" each about 1/10. Sometimes the MFC 
timeout will be longer or shorter than I would otherwise choose in order 
to exclude or include the change from an upcoming stable/ branch.

For context: git log | grep -i -E 'MFC.*after.*:' | sed -E -e 's/^ 
*(X-?)?//i' -e 's/^MFC[a-z0-9]*[- ]after: */MFC after: /i' | sort -fb | 
uniq -ci | sort -bf -k 1nr
19169 MFC after: 1 Week
11807 MFC after: 3 Days
10941 MFC after: 2 Weeks
4002 MFC after: 1 Month
1919 MFC after: 3 Weeks
  838 MFC after: 5 days
  643 MFC after: 2 Days
  621 MFC after: 1 day
  407 MFC after: 4 weeks
  346 MFC after: 2 months
  322 MFC after: 7 days
  282 MFC after: 4 days
  281 MFC after: 10 daysbnb
  222 MFC after: 3 Months
(cutting off at <1% of the most common hold time) for all committers is 
similar to Ed Maste's.

So what do others think? If and when consensus emerges, I'll write it up 
for possible inclusion in the committer's guide.

-- 
#BlackLivesMatter #TransWomenAreWomen #AccessibilityMatters 
#StandWithUkrainians
English: he/him/his (singular they/them/their/theirs OK)
French: il/le/lui (iel/iel and ielle/ielle OK)
Tagalog: siya/niya/kaniya (please avoid sila/nila/kanila)