clang-format, doc: Add guidelines for comments in closing braces and fix code accordingly

Eduardo Almeidarequested to merge
edalm/ns-3-dev:trailing-comments-closing-braces into master

Add guidelines for comments in closing braces. Fix ns-3 code according to these guidelines, either removing ending comments or moving useful comments to separate lines.

This MR is part of the roadmap to enable support of clang-format-18 and beyond, which is tracked in !1899 (merged).

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