meta: note in backport guide about updating tree

[vsemozhetbyt]

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, lts, meta

I am not sure if this is a right thing, please correct me in any way. This is mostly a strawman to save an issue step.

Previously, I've backported PRs only after some weeks after the landing, so in this period I've managed to update my local fork many times. Today I've got a request to backport a PR after just 2 days after the landing. Following the howto steps, I've got this error:

$ git cherry-pick 93d113510d0353d66d72bfd67e338e9ad6031523
fatal: bad object 93d113510d0353d66d72bfd67e338e9ad6031523

It turned out I had not updated my repo during these 2 days. After updating, this error has disappeared.

So maybe this pre-step is worth mentioning for cases of small PRs we now land and backport quicker.

I've just copied the command from COLLABORATOR_GUIDE.md howto, but I am not a git pro, so I may be wrong here.

Sorry for the big diff due to numerating shift.