gn: Mention complete_static_lib in `gn help static_library`.
gn: Mention complete_static_lib in `gn help static_library`.

BUG=none

Review-Url: https://codereview.chromium.org/1975393003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393681}

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