[llvm] [docs] Stress out the branch naming scheme for Graphite. (PR #104499)

[Mehdi Amini via llvm-commits]

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@@ -23,6 +23,13 @@ intended to be able to support "stacked" pull-request. Do not create any branche
 llvm/llvm-project repository otherwise, please use a fork (see below). User branches that
 aren't associated with a pull-request **will be deleted**.
 
+Note: this is particularly necessary if using [Graphite](https://app.graphite.dev/).
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joker-eph wrote:

Introducing Graphite in a "note" seems strange to me. Seems like there should be a section about Graphite itself instead?

https://github.com/llvm/llvm-project/pull/104499