[Docs] Add process for updating musl (#20157)

This adds the process for updating musl to `process.md`.
Drive-by fix of the LLVM git upstream address.

Author: aheejin

docs/process.md

@@ -211,11 +211,11 @@ To update our libraries to a newer LLVM release:
    cases to run [`push_llvm_changes.py`][push_llvm_changes_emscripten] first to
    ensure that no emscripten changes are lost in the process.
 1. Create a PR to merge new LLVM release tag in the upstream repo into our new
-   library branch. For example, if we want to merge LLVM 16.0.6 release into our
-   `emscripten-libs-16` branch, you can do
+   library branch. For example, if we want to merge `llvmorg-16.0.6` tag into
+   our `emscripten-libs-16` branch, you can do
    ```
    git co emscripten-libs-16
-   git remote add upstream https://github.com/llvm/llvm-project
+   git remote add upstream git@github.com:llvm/llvm-project.git
    git fetch --tags upstream
    git merge llvmorg-16.0.6
    ```
@@ -234,6 +234,39 @@ To update our libraries to a newer LLVM release:
    An example of such PR is emscripten-core/emscripten#19515.
 
 
+## Updating musl
+
+We maintain our musl in
+https://github.com/emscripten-core/emscripten/tree/main/system/lib/libc/musl.
+We maintain a fork of musl in https://github.com/emscripten-core/musl for
+updates and periodically update it to a newer version.
+
+The process for updating musl is similar to that of updating the LLVM libraries.
+To update our libraries to a newer musl release:
+
+1. If you are updating an existing branch the first step is to run
+   [`push_musl_changes.py`][push_musl_changes_emscripten] to make sure the
+   current branch is up-to-date with the current emscripten codebase.
+
+   If you are creating a new branch, first make sure the previous/existing
+   branch is up-to-date using
+   [`push_musl_changes.py`][push_musl_changes_emscripten]. Then
+   create the new branch and cherry-pick all the emscripten-specific changes
+   from the old branch, resolving any conflicts that might arise.
+1. Create a PR to merge new mrelease tag in the upstream repo into our new
+   library branch. For example, if we want to merge musl's `v1.2.4` tag into our
+   `merge-v1.2.4` branch, you can do
+   ```
+   git co merge-v1.2.4
+   git remote add upstream git://git.musl-libc.org/musl
+   git fetch --tags upstream
+   git merge v1.2.4
+   ```
+1. Now we have merged all the changes to our musl fork branch, pull those
+   changes with the new version back into the Emscripten repo. You can use
+   [`update_musl.py`][update_musl_emscripten] for that.
+
+
 [site_repo]: https://github.com/kripken/emscripten-site
 [releases_repo]: https://chromium.googlesource.com/emscripten-releases
 [waterfall]: https://ci.chromium.org/p/emscripten-releases/g/main/console
@@ -253,7 +286,9 @@ To update our libraries to a newer LLVM release:
 [llvm_repo]: https://github.com/llvm/llvm-project
 [llvm_emscripten_fork]: https://github.com/emscripten-core/llvm-project
 [push_llvm_changes_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/push_llvm_changes.py
+[push_musl_changes_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/push_musl_changes.py
 [update_compiler_rt_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/update_compiler_rt.py
 [update_libcxx_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/update_libcxx.py
 [update_libcxxabi_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/update_libcxxabi.py
 [update_libunwind_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/update_libunwind.py
+[update_musl_emscripten]: https://github.com/emscripten-core/emscripten/blob/main/system/lib/update_musl.py