[doc] Clarify steps for contributors without commit access.
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@290767 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm