Add more explanations to the github PR template

I don't care about what happens on github, the git tree is what is
important in terms of commit messages.

Add a section on splitting changes up. Each change should do just one
thing. If it does more than one thing, most likely it should be in
a separate commit.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

.github/pull_request_template.md

@@ -31,17 +31,27 @@ notification, use `[GIT PULL]` as a prefix in your PR title.
 ### Commit message format rules:
 1. The first line is title (don't be more than 72 chars if possible).
 2. Then an empty line.
-3. Then a description (may be omitted for trivial changes).
+3. Then a description (may be omitted for truly trivial changes).
 4. Then an empty line again (if it has a description).
 5. Then a `Signed-off-by` tag with your real name and email. For example:
 ```
 Signed-off-by: Foo Bar <foo.bar@gmail.com>
 ```
 
-Description should be word-wrapped at 72 chars. Some things should not
-be word-wrapped. They may be some kind of quoted text - long compiler
-error messages, oops reports, Link, etc. (things that have a certain
-specific format).
+The description should be word-wrapped at 72 chars. Some things should
+not be word-wrapped. They may be some kind of quoted text - long
+compiler error messages, oops reports, Link, etc. (things that have a
+certain specific format).
+
+Note that all of this goes in the commit message, not in the pull
+request text. The pull request text should introduce what this pull
+request does, and each commit message should explain the rationale for
+why that particular change was made. The git tree is canonical source
+of truth, not github.
+
+Each patch should do one thing, and one thing only. If you find yourself
+writing an explanation for why a patch is fixing multiple issues, that's
+a good indication that the change should be split into separate patches.
 
 If the commit is a fix for an issue, add a `Fixes` tag with the issue
 URL.