Small change to docs for module import syntax

[DilumAluthge]

Note: the base branch (target branch) of this PR is kc/warn_using (#42080), NOT master.

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This small change was originally part of #53428. However, it caused confusion during PR review. So I removed this change from #53428 (in https://github.com/JuliaLang/julia/pull/53428/commits/862a5466ba19acee25d566488431123fe5494ca1), and I've put that change into this PR.

Just to clarify, the language about "keeping the namespace clean" already existed on master prior to #53428 (and prior to this PR). All this PR does is:

1. Move the existing "clean namespace" sentence down.
2. In that existing sentence, change "import ModuleName" to "import ModuleName or using ModuleName: ModuleName".