docs: clarify "plumbing" v.s. "porcelain" in git(1) usage

Clarify the recently added documentation in 2cbfe50d7 (docs: Explain
how to interact with Git in Gitaly, 2020-09-23) to not draw a hard
line on using non-plumbing. As the change being made here argues this
doesn't reflect reality on the ground.

As a practical matter we make plenty of use of non-plumbing,
e.g. "gc", "bundle", "log", "remote", "config", "worktree" etc. What
we actually care about is the sort of command use I altered in
61b9b30a5 (git.Remote: use "git remote" exit code instead of parsing
stderr, 2020-11-10). Let's try to make the docs reflect that.

I also sent a note to the git ML about the general issue:
https://public-inbox.org/git/878sa7xujm.fsf@evledraar.gmail.com/T/

1 files changed, 12 insertions, 2 deletions

diff --git a/doc/serverside_git_usage.md b/doc/serverside_git_usage.md
index 9a4d464da..a3d36e7c7 100644
--- a/doc/serverside_git_usage.md
+++ b/doc/serverside_git_usage.md
@@ -18,8 +18,18 @@ commands. Porcelain commands are intended for the end-user and are the
 user-interface of the default `git` client, where plumbing commands
 are intended for scripted use or to build another porcelain.
 
-Gitaly should only use plumbing commands. `man 1 git` contains a
-section on the low level plumbing.
+Generally speaking, Gitaly should only use plumbing commands. `man 1
+git` contains a section on the low level plumbing. However, a lot of
+git's plumbing-like functionality is exposed as commands not marked as
+plumbing, but whose API reliability can be considered the
+same. E.g. `git log`'s `--pretty=` formats, `git config -l -z`, the
+documented exit codes of `git remote` etc..
+
+We should use good judgement when choosing what commands and command
+functionality to use, with the aim of not having gitaly break due to
+e.g. an error message being rephrased or functionality the upstream
+`git` maintainers don't consider plumbing-like being removed or
+altered.
 
 #### Executing Git commands