Fix some trivial but fatal warnings generated by clang when run under a
Windows MSYS2 build. Most are `-Wdocumentation` warnings caused by
docstring comments slipping out of sync with the code they document.
Remove unused variable `i_handles` from `sync_pipe_start()` in
`capture/capture_sync.c` to prevent a `-Wunused-but-set` error.
Use `DIAG_OFF(unreachable-code)` in `wsutil/test_wsutil.c` to prevent a
fully-expected `Wunreachable-code` "error" caused by short-circuiting
the function in Windows.
All changes have been confirmed to have no ill effect on a MSVC Windows
build. Changes should not negatively impact other platforms but this has
not been confirmed.
5f485d59c9