docs: avoid ambiguity between update and fetch (#15860)

address
<#15853 (comment)>

r? epage

Author: epage

src/doc/src/reference/specifying-dependencies.md

@@ -307,13 +307,11 @@ When cloning a `git` dependency,
 Cargo automatically fetches its submodules recursively
 so that all required code is available for the build.
 
-To skip updates for a specific submodule,
-Cargo respects the [`submodule.<name>.update`] setting in `.gitmodules`.
-Setting it to `none` disables updates for that submodule if it isn't needed for the build.
-This is usually set in the repository itself,
-so changes require access to the dependency's repo.
+To skip fetching submodules unrelated to the build,
+you can set [`submodule.<name>.update = none`][submodule-update] in the dependency repo's `.gitmodules`.
+This requires write access to the repo and will disable submodule updates more generally.
 
-[`submodule.<name>.update`]: https://git-scm.com/docs/gitmodules#Documentation/gitmodules.txt-submodulenameupdate
+[submodule-update]: https://git-scm.com/docs/gitmodules#Documentation/gitmodules.txt-submodulenameupdate
 
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