[doc] Clarify steps for contributors without commit access.

Summary: Update the Phabricator docs to clarify how changes are merged for contributors without commit access. 

Reviewers: delcypher, aaron.ballman

Subscribers: aaron.ballman, anmol, llvm-commits

Differential Revision: https://reviews.llvm.org/D28184

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290767 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed