[llvm] docs: Clarify commit access requirements in the Developer Policy (PR #101414)

[Shilei Tian via llvm-commits]

================
@@ -475,8 +475,8 @@ What are the expectations around a revert?
 Obtaining Commit Access
 -----------------------
 
-We grant commit access to contributors with a track record of submitting high
-quality patches.  If you would like commit access, please send an email to
+We grant commit access to contributors that can provide a valid justification
----------------
shiltian wrote:

```suggestion
We grant commit access to contributors that can provide a valid justification.
```

https://github.com/llvm/llvm-project/pull/101414